Zacharek SJ, Fillmore CM, Lau AN, Gludish DW, Chou A, Ho JWK, Zamponi R, Gazit R, Bock C, Jäger N, Smith ZD, Kim T-M, Saunders AH, Wong J, Lee J-H, Roach RR, Rossi DJ, Meissner A, Gimelbrant AA, Park PJ, Kim CF. Lung stem cell self-renewal relies on BMI1-dependent control of expression at imprinted loci. Cell Stem Cell 2011;9(3):272-81.Abstract

BMI1 is required for the self-renewal of stem cells in many tissues including the lung epithelial stem cells, Bronchioalveolar Stem Cells (BASCs). Imprinted genes, which exhibit expression from only the maternally or paternally inherited allele, are known to regulate developmental processes, but what their role is in adult cells remains a fundamental question. Many imprinted genes were derepressed in Bmi1 knockout mice, and knockdown of Cdkn1c (p57) and other imprinted genes partially rescued the self-renewal defect of Bmi1 mutant lung cells. Expression of p57 and other imprinted genes was required for lung cell self-renewal in culture and correlated with repair of lung epithelial cell injury in vivo. Our data suggest that BMI1-dependent regulation of expressed alleles at imprinted loci, distinct from imprinting per se, is required for control of lung stem cells. We anticipate that the regulation and function of imprinted genes is crucial for self-renewal in diverse adult tissue-specific stem cells.

Plasticity in patterns of histone modifications and chromosomal proteins in Drosophila heterochromatin.
Riddle NC*, Minoda A*, Kharchenko PV*, Alekseyenko AA, Schwartz YB, Tolstorukov MY, Gorchakov AA, Jaffe JD, Kennedy C, Linder-Basso D, Peach SE, Shanower G, Zheng H, Kuroda MI, Pirrotta V, Park PJ, Elgin SCR**, Karpen GH**. Plasticity in patterns of histone modifications and chromosomal proteins in Drosophila heterochromatin. Genome Res 2011;21(2):147-63.Abstract

Eukaryotic genomes are packaged in two basic forms, euchromatin and heterochromatin. We have examined the composition and organization of Drosophila melanogaster heterochromatin in different cell types using ChIP-array analysis of histone modifications and chromosomal proteins. As anticipated, the pericentric heterochromatin and chromosome 4 are on average enriched for the "silencing" marks H3K9me2, H3K9me3, HP1a, and SU(VAR)3-9, and are generally depleted for marks associated with active transcription. The locations of the euchromatin-heterochromatin borders identified by these marks are similar in animal tissues and most cell lines, although the amount of heterochromatin is variable in some cell lines. Combinatorial analysis of chromatin patterns reveals distinct profiles for euchromatin, pericentric heterochromatin, and the 4th chromosome. Both silent and active protein-coding genes in heterochromatin display complex patterns of chromosomal proteins and histone modifications; a majority of the active genes exhibit both "activation" marks (e.g., H3K4me3 and H3K36me3) and "silencing" marks (e.g., H3K9me2 and HP1a). The hallmark of active genes in heterochromatic domains appears to be a loss of H3K9 methylation at the transcription start site. We also observe complex epigenomic profiles of intergenic regions, repeated transposable element (TE) sequences, and genes in the heterochromatic extensions. An unexpectedly large fraction of sequences in the euchromatic chromosome arms exhibits a heterochromatic chromatin signature, which differs in size, position, and impact on gene expression among cell types. We conclude that patterns of heterochromatin/euchromatin packaging show greater complexity and plasticity than anticipated. This comprehensive analysis provides a foundation for future studies of gene activity and chromosomal functions that are influenced by or dependent upon heterochromatin.

Larschan E*, Bishop EP*, Kharchenko PV, Core LJ, Lis JT, Park PJ**, Kuroda MI**. X chromosome dosage compensation via enhanced transcriptional elongation in Drosophila. Nature 2011;471(7336):115-8.Abstract

The evolution of sex chromosomes has resulted in numerous species in which females inherit two X chromosomes but males have a single X, thus requiring dosage compensation. MSL (Male-specific lethal) complex increases transcription on the single X chromosome of Drosophila males to equalize expression of X-linked genes between the sexes. The biochemical mechanisms used for dosage compensation must function over a wide dynamic range of transcription levels and differential expression patterns. It has been proposed that the MSL complex regulates transcriptional elongation to control dosage compensation, a model subsequently supported by mapping of the MSL complex and MSL-dependent histone 4 lysine 16 acetylation to the bodies of X-linked genes in males, with a bias towards 3' ends. However, experimental analysis of MSL function at the mechanistic level has been challenging owing to the small magnitude of the chromosome-wide effect and the lack of an in vitro system for biochemical analysis. Here we use global run-on sequencing (GRO-seq) to examine the specific effect of the MSL complex on RNA Polymerase II (RNAP II) on a genome-wide level. Results indicate that the MSL complex enhances transcription by facilitating the progression of RNAP II across the bodies of active X-linked genes. Improving transcriptional output downstream of typical gene-specific controls may explain how dosage compensation can be imposed on the diverse set of genes along an entire chromosome.

Ho JWK, Bishop EP, Karchenko PV, Nègre N, White KP, Park PJ. ChIP-chip versus ChIP-seq: lessons for experimental design and data analysis. BMC Genomics 2011;12:134.Abstract

BACKGROUND: Chromatin immunoprecipitation (ChIP) followed by microarray hybridization (ChIP-chip) or high-throughput sequencing (ChIP-seq) allows genome-wide discovery of protein-DNA interactions such as transcription factor bindings and histone modifications. Previous reports only compared a small number of profiles, and little has been done to compare histone modification profiles generated by the two technologies or to assess the impact of input DNA libraries in ChIP-seq analysis. Here, we performed a systematic analysis of a modENCODE dataset consisting of 31 pairs of ChIP-chip/ChIP-seq profiles of the coactivator CBP, RNA polymerase II (RNA PolII), and six histone modifications across four developmental stages of Drosophila melanogaster. RESULTS: Both technologies produce highly reproducible profiles within each platform, ChIP-seq generally produces profiles with a better signal-to-noise ratio, and allows detection of more peaks and narrower peaks. The set of peaks identified by the two technologies can be significantly different, but the extent to which they differ varies depending on the factor and the analysis algorithm. Importantly, we found that there is a significant variation among multiple sequencing profiles of input DNA libraries and that this variation most likely arises from both differences in experimental condition and sequencing depth. We further show that using an inappropriate input DNA profile can impact the average signal profiles around genomic features and peak calling results, highlighting the importance of having high quality input DNA data for normalization in ChIP-seq analysis. CONCLUSIONS: Our findings highlight the biases present in each of the platforms, show the variability that can arise from both technology and analysis methods, and emphasize the importance of obtaining high quality and deeply sequenced input DNA libraries for ChIP-seq analysis.

Kharchenko PV, Alekseyenko AA, Schwartz YB, Minoda A, Riddle NC, Ernst J, Sabo PJ, Larschan E, Gorchakov AA, Gu T, Linder-Basso D, Plachetka A, Shanower G, Tolstorukov MY, Luquette LJ, Xi R, Jung YL, Park RW, Bishop EP, Canfield TK, Sandstrom R, Thurman RE, MacAlpine DM, Stamatoyannopoulos JA, Kellis M, Elgin SCR, Kuroda MI, Pirrotta V, Karpen GH**, Park PJ**. Comprehensive analysis of the chromatin landscape in Drosophila melanogaster. Nature 2011;471(7339):480-5.Abstract

Chromatin is composed of DNA and a variety of modified histones and non-histone proteins, which have an impact on cell differentiation, gene regulation and other key cellular processes. Here we present a genome-wide chromatin landscape for Drosophila melanogaster based on eighteen histone modifications, summarized by nine prevalent combinatorial patterns. Integrative analysis with other data (non-histone chromatin proteins, DNase I hypersensitivity, GRO-Seq reads produced by engaged polymerase, short/long RNA products) reveals discrete characteristics of chromosomes, genes, regulatory elements and other functional domains. We find that active genes display distinct chromatin signatures that are correlated with disparate gene lengths, exon patterns, regulatory functions and genomic contexts. We also demonstrate a diversity of signatures among Polycomb targets that include a subset with paused polymerase. This systematic profiling and integrative analysis of chromatin signatures provides insights into how genomic elements are regulated, and will serve as a resource for future experimental investigations of genome structure and function.

Tolstorukov MY, Volfovsky N, Stephens RM, Park PJ. Impact of chromatin structure on sequence variability in the human genome. Nat Struct Mol Biol 2011;18(4):510-5.Abstract

DNA sequence variations in individual genomes give rise to different phenotypes within the same species. One mechanism in this process is the alteration of chromatin structure due to sequence variation that influences gene regulation. We composed a high-confidence collection of human single-nucleotide polymorphisms and indels based on analysis of publicly available sequencing data and investigated whether the DNA loci associated with stable nucleosome positions are protected against mutations. We addressed how the sequence variation reflects the occupancy profiles of nucleosomes bearing different epigenetic modifications on genome scale. We found that indels are depleted around nucleosome positions of all considered types, whereas single-nucleotide polymorphisms are enriched around the positions of bulk nucleosomes but depleted around the positions of epigenetically modified nucleosomes. These findings indicate an increased level of conservation for the sequences associated with epigenetically modified nucleosomes, highlighting complex organization of the human chromatin.

Blackledge NP, Zhou JC, Tolstorukov MY, Farcas AM, Park PJ, Klose RJ. CpG islands recruit a histone H3 lysine 36 demethylase. Molecular Cell 2010;38(2):179-90.Abstract

In higher eukaryotes, up to 70% of genes have high levels of nonmethylated cytosine/guanine base pairs (CpGs) surrounding promoters and gene regulatory units. These features, called CpG islands, were identified over 20 years ago, but there remains little mechanistic evidence to suggest how these enigmatic elements contribute to promoter function, except that they are refractory to epigenetic silencing by DNA methylation. Here we show that CpG islands directly recruit the H3K36-specific lysine demethylase enzyme KDM2A. Nucleation of KDM2A at these elements results in removal of H3K36 methylation, creating CpG island chromatin that is uniquely depleted of this modification. KDM2A utilizes a zinc finger CxxC (ZF-CxxC) domain that preferentially recognizes nonmethylated CpG DNA, and binding is blocked when the CpG DNA is methylated, thus constraining KDM2A to nonmethylated CpG islands. These data expose a straightforward mechanism through which KDM2A delineates a unique architecture that differentiates CpG island chromatin from bulk chromatin.

modENCODE Consortium *, Roy S*, Ernst J*, Kharchenko PV*, Kheradpour P*, Negre N*, Eaton ML*, Landolin JM*, Bristow CA*, Ma L*, Lin MF*, Washietl S*, Arshinoff BI, Ay F, Meyer PE, Robine N, Washington NL, Di Stefano L, Berezikov E, Brown CD, Candeias R, Carlson JW, Carr A, Jungreis I, Marbach D, Sealfon R, Tolstorukov MY, Will S, Alekseyenko AA, Artieri C, Booth BW, Brooks AN, Dai Q, Davis CA, Duff MO, Feng X, Gorchakov AA, Gu T, Henikoff JG, Kapranov P, Li R, MacAlpine HK, Malone J, Minoda A, Nordman J, Okamura K, Perry M, Powell SK, Riddle NC, Sakai A, Samsonova A, Sandler JE, Schwartz YB, Sher N, Spokony R, Sturgill D, van Baren M, Wan KH, Yang L, Yu C, Feingold E, Good P, Guyer M, Lowdon R, Ahmad K, Andrews J, Berger B, Brenner SE, Brent MR, Cherbas L, Elgin SCR, Gingeras TR, Grossman R, Hoskins RA, Kaufman TC, Kent W, Kuroda MI, Orr-Weaver T, Perrimon N, Pirrotta V, Posakony JW, Ren B, Russell S, Cherbas P, Graveley BR, Lewis S, Micklem G, Oliver B, Park PJ, Celniker SE**, Henikoff S**, Karpen GH**, Lai EC**, MacAlpine DM**, Stein LD**, White KP**, Kellis M**. Identification of functional elements and regulatory circuits by Drosophila modENCODE. Science 2010;330(6012):1787-97.Abstract

To gain insight into how genomic information is translated into cellular and developmental programs, the Drosophila model organism Encyclopedia of DNA Elements (modENCODE) project is comprehensively mapping transcripts, histone modifications, chromosomal proteins, transcription factors, replication proteins and intermediates, and nucleosome properties across a developmental time course and in multiple cell lines. We have generated more than 700 data sets and discovered protein-coding, noncoding, RNA regulatory, replication, and chromatin elements, more than tripling the annotated portion of the Drosophila genome. Correlated activity patterns of these elements reveal a functional regulatory network, which predicts putative new functions for genes, reveals stage- and tissue-specific regulators, and enables gene-expression prediction. Our results provide a foundation for directed experimental and computational studies in Drosophila and related species and also a model for systematic data integration toward comprehensive genomic and functional annotation.

Peng S, Kuroda MI, Park PJ. Quantized correlation coefficient for measuring reproducibility of ChIP-chip data. BMC Bioinformatics 2010;11:399.Abstract

BACKGROUND: Chromatin immunoprecipitation followed by microarray hybridization (ChIP-chip) is used to study protein-DNA interactions and histone modifications on a genome-scale. To ensure data quality, these experiments are usually performed in replicates, and a correlation coefficient between replicates is used often to assess reproducibility. However, the correlation coefficient can be misleading because it is affected not only by the reproducibility of the signal but also by the amount of binding signal present in the data. RESULTS: We develop the Quantized correlation coefficient (QCC) that is much less dependent on the amount of signal. This involves discretization of data into set of quantiles (quantization), a merging procedure to group the background probes, and recalculation of the Pearson correlation coefficient. This procedure reduces the influence of the background noise on the statistic, which then properly focuses more on the reproducibility of the signal. The performance of this procedure is tested in both simulated and real ChIP-chip data. For replicates with different levels of enrichment over background and coverage, we find that QCC reflects reproducibility more accurately and is more robust than the standard Pearson or Spearman correlation coefficients. The quantization and the merging procedure can also suggest a proper quantile threshold for separating signal from background for further analysis. CONCLUSIONS: To measure reproducibility of ChIP-chip data correctly, a correlation coefficient that is robust to the amount of signal present should be used. QCC is one such measure. The QCC statistic can also be applied in a variety of other contexts for measuring reproducibility, including analysis of array CGH data for DNA copy number and gene expression data.

Woo CJ, Kharchenko PV, Daheron L, Park PJ, Kingston RE. A region of the human HOXD cluster that confers polycomb-group responsiveness. Cell 2010;140(1):99-110.Abstract

Polycomb group (PcG) proteins are essential for accurate axial body patterning during embryonic development. PcG-mediated repression is conserved in metazoans and is targeted in Drosophila by Polycomb response elements (PREs). However, targeting sequences in humans have not been described. While analyzing chromatin architecture in the context of human embryonic stem cell (hESC) differentiation, we discovered a 1.8kb region between HOXD11 and HOXD12 (D11.12) that is associated with PcG proteins, becomes nuclease hypersensitive, and then shows alteration in nuclease sensitivity as hESCs differentiate. The D11.12 element repressed luciferase expression from a reporter construct and full repression required a highly conserved region and YY1 binding sites. Furthermore, repression was dependent on the PcG proteins BMI1 and EED and a YY1-interacting partner, RYBP. We conclude that D11.12 is a Polycomb-dependent regulatory region with similarities to Drosophila PREs, indicating conservation in the mechanisms that target PcG function in mammals and flies.

Tolstorukov MY, Kharchenko PV, Park PJ. Analysis of primary structure of chromatin with next-generation sequencing. Epigenomics 2010;2(2):187-197.Abstract

The recent development of next-generation sequencing technology has enabled significant progress in chromatin structure analysis. Here, we review the experimental and bioinformatic approaches to studying nucleosome positioning and histone modification profiles on a genome scale using this technology. These studies advanced our knowledge of the nucleosome positioning patterns of both epigenetically modified and bulk nucleosomes and elucidated the role of such patterns in regulation of gene expression. The identification and analysis of large sets of nucleosome-bound DNA sequences allowed better understanding of the rules that govern nucleosome positioning in organisms of various complexity. We also discuss the existing challenges and prospects of using next-generation sequencing for nucleosome positioning analysis and outline the importance of such studies for the entire chromatin structure field.

Gelbart ME, Larschan E, Peng S, Park PJ, Kuroda MI. Drosophila MSL complex globally acetylates H4K16 on the male X chromosome for dosage compensation. Nat Struct Mol Biol 2009;16(8):825-32.Abstract

The Drosophila melanogaster male-specific lethal (MSL) complex binds the single male X chromosome to upregulate gene expression to equal that from the two female X chromosomes. However, it has been puzzling that approximately 25% of transcribed genes on the X chromosome do not stably recruit MSL complex. Here we find that almost all active genes on the X chromosome are associated with robust H4 Lys16 acetylation (H4K16ac), the histone modification catalyzed by the MSL complex. The distribution of H4K16ac is much broader than that of the MSL complex, and our results favor the idea that chromosome-wide H4K16ac reflects transient association of the MSL complex, occurring through spreading or chromosomal looping. Our results parallel those of localized Polycomb repressive complex and its more broadly distributed chromatin mark, trimethylated histone H3 Lys27 (H3K27me3), suggesting a common principle for the establishment of active and silenced chromatin domains.

Gorchakov AA, Alekseyenko AA, Kharchenko P, Park PJ, Kuroda MI. Long-range spreading of dosage compensation in Drosophila captures transcribed autosomal genes inserted on X. Genes Dev 2009;23(19):2266-71.Abstract

Dosage compensation in Drosophila melanogaster males is achieved via targeting of male-specific lethal (MSL) complex to X-linked genes. This is proposed to involve sequence-specific recognition of the X at approximately 150-300 chromatin entry sites, and subsequent spreading to active genes. Here we ask whether the spreading step requires transcription and is sequence-independent. We find that MSL complex binds, acetylates, and up-regulates autosomal genes inserted on X, but only if transcriptionally active. We conclude that a long-sought specific DNA sequence within X-linked genes is not obligatory for MSL binding. Instead, linkage and transcription play the pivotal roles in MSL targeting irrespective of gene origin and DNA sequence.

Park PJ. ChIP-seq: advantages and challenges of a maturing technology. Nat Rev Genet 2009;10(10):669-80.Abstract

Chromatin immunoprecipitation followed by sequencing (ChIP-seq) is a technique for genome-wide profiling of DNA-binding proteins, histone modifications or nucleosomes. Owing to the tremendous progress in next-generation sequencing technology, ChIP-seq offers higher resolution, less noise and greater coverage than its array-based predecessor ChIP-chip. With the decreasing cost of sequencing, ChIP-seq has become an indispensable tool for studying gene regulation and epigenetic mechanisms. In this Review, I describe the benefits and challenges in harnessing this technique with an emphasis on issues related to experimental design and data analysis. ChIP-seq experiments generate large quantities of data, and effective computational analysis will be crucial for uncovering biological mechanisms.

Comparative analysis of H2A.Z nucleosome organization in the human and yeast genomes.
Tolstorukov MY*, Kharchenko PV*, Goldman JA, Kingston RE, Park PJ. Comparative analysis of H2A.Z nucleosome organization in the human and yeast genomes. Genome Res 2009;19(6):967-77.Abstract

Eukaryotic DNA is wrapped around a histone protein core to constitute the fundamental repeating units of chromatin, the nucleosomes. The affinity of the histone core for DNA depends on the nucleotide sequence; however, it is unclear to what extent DNA sequence determines nucleosome positioning in vivo, and if the same rules of sequence-directed positioning apply to genomes of varying complexity. Using the data generated by high-throughput DNA sequencing combined with chromatin immunoprecipitation, we have identified positions of nucleosomes containing the H2A.Z histone variant and histone H3 trimethylated at lysine 4 in human CD4(+) T-cells. We find that the 10-bp periodicity observed in nucleosomal sequences in yeast and other organisms is not pronounced in human nucleosomal sequences. This result was confirmed for a broader set of mononucleosomal fragments that were not selected for any specific histone variant or modification. We also find that human H2A.Z nucleosomes protect only approximately 120 bp of DNA from MNase digestion and exhibit specific sequence preferences, suggesting a novel mechanism of nucleosome organization for the H2A.Z variant.

Orford K*, Kharchenko P*, Lai W, Dao MC, Worhunsky DJ, Ferro A, Janzen V, Park PJ**, Scadden DT**. Differential H3K4 methylation identifies developmentally poised hematopoietic genes. Dev Cell 2008;14(5):798-809.Abstract

Throughout development, cell fate decisions are converted into epigenetic information that determines cellular identity. Covalent histone modifications are heritable epigenetic marks and are hypothesized to play a central role in this process. In this report, we assess the concordance of histone H3 lysine 4 dimethylation (H3K4me2) and trimethylation (H3K4me3) on a genome-wide scale in erythroid development by analyzing pluripotent, multipotent, and unipotent cell types. Although H3K4me2 and H3K4me3 are concordant at most genes, multipotential hematopoietic cells have a subset of genes that are differentially methylated (H3K4me2+/me3-). These genes are transcriptionally silent, highly enriched in lineage-specific hematopoietic genes, and uniquely susceptible to differentiation-induced H3K4 demethylation. Self-renewing embryonic stem cells, which restrict H3K4 methylation to genes that contain CpG islands (CGIs), lack H3K4me2+/me3- genes. These data reveal distinct epigenetic regulation of CGI and non-CGI genes during development and indicate an interactive relationship between DNA sequence and differential H3K4 methylation in lineage-specific differentiation.

Park PJ. Epigenetics meets next-generation sequencing. Epigenetics 2008;3(6):318-21.Abstract

Next-generation sequencing is poised to unleash dramatic changes in every area of molecular biology. In the past few years, chromatin immunoprecipitation (ChIP) on tiled microarrays (ChIP-chip) has been an important tool for genome-wide mapping of DNA-binding proteins or histone modifications. Now, ChIP followed by direct sequencing of DNA fragments (ChIP-seq) offers superior data with less noise and higher resolution and is likely to replace ChIP-chip in the near future. We will describe advantages of this new technology and outline some of the issues in dealing with the data. ChIP-seq generates considerably larger quantities of data and the most challenging aspect for investigators will be computational and statistical analysis necessary to uncover biological insights hidden in the data.

Kharchenko PV*, Woo CJ*, Tolstorukov MY, Kingston RE**, Park PJ**. Nucleosome positioning in human HOX gene clusters. Genome Res 2008;18(10):1554-61.Abstract

The distribution of nucleosomes along the genome is a significant aspect of chromatin structure and is thought to influence gene regulation through modulation of DNA accessibility. However, properties of nucleosome organization remain poorly understood, particularly in mammalian genomes. Toward this goal we used tiled microarrays to identify stable nucleosome positions along the HOX gene clusters in human cell lines. We show that nucleosome positions exhibit sequence properties and long-range organization that are different from those characterized in other organisms. Despite overall variability of internucleosome distances, specific loci contain regular nucleosomal arrays with 195-bp periodicity. Moreover, such arrays tend to occur preferentially toward the 3' ends of genes. Through comparison of different cell lines, we find that active transcription is correlated with increased positioning of nucleosomes, suggesting an unexpected role for transcription in the establishment of well-positioned nucleosomes.

High-resolution ChIP-chip analysis reveals that the Drosophila MSL complex selectively identifies active genes on the male X chromosome.
Alekseyenko AA, Larschan E, Lai WR, Park PJ**, Kuroda MI**. High-resolution ChIP-chip analysis reveals that the Drosophila MSL complex selectively identifies active genes on the male X chromosome. Genes Dev 2006;20(7):848-57.Abstract

X-chromosome dosage compensation in Drosophila requires the male-specific lethal (MSL) complex, which up-regulates gene expression from the single male X chromosome. Here, we define X-chromosome-specific MSL binding at high resolution in two male cell lines and in late-stage embryos. We find that the MSL complex is highly enriched over most expressed genes, with binding biased toward the 3' end of transcription units. The binding patterns are largely similar in the distinct cell types, with approximately 600 genes clearly bound in all three cases. Genes identified as clearly bound in one cell type and not in another indicate that attraction of MSL complex correlates with expression state. Thus, sequence alone is not sufficient to explain MSL targeting. We propose that the MSL complex recognizes most X-linked genes, but only in the context of chromatin factors or modifications indicative of active transcription. Distinguishing expressed genes from the bulk of the genome is likely to be an important function common to many chromatin organizing and modifying activities.

Namekawa SH, Park PJ, Zhang L-F, Shima JE, McCarrey JR, Griswold MD, Lee JT. Postmeiotic sex chromatin in the male germline of mice. Curr Biol 2006;16(7):660-7.Abstract

In mammals, the X and Y chromosomes are subject to meiotic sex chromosome inactivation (MSCI) during prophase I in the male germline, but their status thereafter is currently unclear. An abundance of X-linked spermatogenesis genes has spawned the view that the X must be active . On the other hand, the idea that the imprinted paternal X of the early embryo may be preinactivated by MSCI suggests that silencing may persist longer . To clarify this issue, we establish a comprehensive X-expression profile during mouse spermatogenesis. Here, we discover that the X and Y occupy a novel compartment in the postmeiotic spermatid and adopt a non-Rabl configuration. We demonstrate that this postmeiotic sex chromatin (PMSC) persists throughout spermiogenesis into mature sperm and exhibits epigenetic similarity to the XY body. In the spermatid, 87% of X-linked genes remain suppressed postmeiotically, while autosomes are largely active. We conclude that chromosome-wide X silencing continues from meiosis to the end of spermiogenesis, and we discuss implications for proposed mechanisms of imprinted X-inactivation.