A ChIP-exo screen of 887 Protein Capture Reagents Program transcription factor antibodies in human cells.

Antibodies offer a powerful means to interrogate specific proteins in a complex milieu. However, antibody availability and reliability can be problematic, whereas epitope tagging can be impractical in many cases. To address these limitations, the Protein Capture Reagents Program (PCRP) generated over a thousand renewable monoclonal antibodies (mAbs) against human presumptive chromatin proteins. However, these reagents have not been widely field-tested. We therefore performed a screen to test their ability to enrich genomic regions via chromatin immunoprecipitation (ChIP) and a variety of orthogonal assays. Eight hundred eighty-seven unique antibodies against 681 unique human transcription factors (TFs) were assayed by ultra-high-resolution ChIP-exo/seq, generating approximately 1200 ChIP-exo data sets, primarily in a single pass in one cell type (K562). Subsets of PCRP mAbs were further tested in ChIP-seq, CUT&RUN, STORM super-resolution microscopy, immunoblots, and protein binding microarray (PBM) experiments. About 5% of the tested antibodies displayed high-confidence target (i.e., cognate antigen) enrichment across at least one assay and are strong candidates for additional validation. An additional 34% produced ChIP-exo data that were distinct from background and thus warrant further testing. The remaining 61% were not substantially different from background, and likely require consideration of a much broader survey of cell types and/or assay optimizations. We show and discuss the metrics and challenges to antibody validation in chromatin-based assays.

Ultrasonographic diagnosis of adnexal masses: interobserver agreement in the interpretation of videos, using IOTA terminology.

OBJECTIVES: Aim of this study is to estimate interobserver agreement in classifying adnexal tumors using IOTA terms, simple rules and subjective assessment. In addition, we related observers' accuracy with their experience in gynecological ultrasonography and the year of IOTA certification. METHODS: Eleven observers with three different levels of experience evaluated videoclips of 70 adnexal masses, defining tumor type according to IOTA terms and definitions, classifying the mass using IOTA Simple rules and Subjective assessment as well as providing Color Score evaluation. Sensitivity, specificity and area under the ROC curve were calculated and the year of IOTA certification was related with operators' accuracy through Pearson correlation coefficient. Interobserver agreement was estimated calculating percentage of agreement, Fleiss kappa and Cohen's kappa. RESULTS: We found a positive correlation between the year of IOTA certification and operators' accuracy (Pearson coefficient 0.694), especially among the observers with the least experience, the residents (p = 0.003). For tumor type classification, identification of papillary projections and classification of tumors using subjective assessment, agreement among all observers was moderate (Fleiss kappa 0.455, 0.552, and 0.476, respectively) and increased with the years of experience. Agreement in the application of Simple Rules was moderate in all examiners with IOTA certification, with Fleiss kappa in the range of (0.403, 0.498). For Color Score assignment interobserver agreement among all observers was fair (Cohen's kappa 0.380). CONCLUSIONS: Even among expert examiners, the results of adnexal lesion assessment can be inconsistent. Experience impacts on accuracy and agreement in subjective assessment, while the application of Simple Rules can mitigate the role of experience in interobserver agreement. The knowledge of IOTA models among residents seams to improve their diagnostic accuracy, showing the benefits of IOTA terminology for in training sonographers.

MEDEA: analysis of transcription factor binding motifs in accessible chromatin.

Deciphering the interplay between chromatin accessibility and transcription factor (TF) binding is fundamental to understanding transcriptional regulation, control of cellular states, and the establishment of new phenotypes. Recent genome-wide chromatin accessibility profiling studies have provided catalogs of putative open regions, where TFs can recognize their motifs and regulate gene expression programs. Here, we present motif enrichment in differential elements of accessibility (MEDEA), a computational tool that analyzes high-throughput chromatin accessibility genomic data to identify cell-type-specific accessible regions and lineage-specific motifs associated with TF binding therein. To benchmark MEDEA, we used a panel of reference cell lines profiled by ENCODE and curated by the ENCODE Project Consortium for the ENCODE-DREAM Challenge. By comparing results with RNA-seq data, ChIP-seq peaks, and DNase-seq footprints, we show that MEDEA improves the detection of motifs associated with known lineage specifiers. We then applied MEDEA to 610 ENCODE DNase-seq data sets, where it revealed significant motifs even when absolute enrichment was low and where it identified novel regulators, such as NRF1 in kidney development. Finally, we show that MEDEA performs well on both bulk and single-cell ATAC-seq data. MEDEA is publicly available as part of our Glossary-GENRE suite for motif enrichment analysis.

Comparative chromatin accessibility upon BDNF stimulation delineates neuronal regulatory elements.

Neuronal stimulation induced by the brain-derived neurotrophic factor (BDNF) triggers gene expression, which is crucial for neuronal survival, differentiation, synaptic plasticity, memory formation, and neurocognitive health. However, its role in chromatin regulation is unclear. Here, using temporal profiling of chromatin accessibility and transcription in mouse primary cortical neurons upon either BDNF stimulation or depolarization (KCl), we identify features that define BDNF-specific chromatin-to-gene expression programs. Enhancer activation is an early event in the regulatory control of BDNF-treated neurons, where the bZIP motif-binding Fos protein pioneered chromatin opening and cooperated with co-regulatory transcription factors (Homeobox, EGRs, and CTCF) to induce transcription. Deleting cis-regulatory sequences affect BDNF-mediated Arc expression, a regulator of synaptic plasticity. BDNF-induced accessible regions are linked to preferential exon usage by neurodevelopmental disorder-related genes and the heritability of neuronal complex traits, which were validated in human iPSC-derived neurons. Thus, we provide a comprehensive view of BDNF-mediated genome regulatory features using comparative genomic approaches to dissect mammalian neuronal stimulation.

Uterine smooth muscle tumors: a multicenter, retrospective, comparative study of clinical and ultrasound features.

OBJECTIVE: To evaluate a wide range of clinical and ultrasound characteristics of different uterine smooth muscle tumors to identify features capable of discriminating between these types. METHODS: This was a retrospective, multicenter study that included 285 patients diagnosed with uterine smooth muscle tumors (50 leiomyosarcomas, 35 smooth muscle tumors of uncertain malignant potential, and 200 leiomyomas). The patients were divided into three groups based on the histological type of their tumors, and the groups were compared according to the variables collected. RESULTS: Leiomyosarcomas were more common in older and post-menopausal women. Compared with leiomyomas, smooth muscle tumors of uncertain malignant potential and leiomyosarcomas had similar ultrasound features such as absence of normal myometrium, multilocular appearance, hyper-echogenicity in case of uniform echogenicity, absence of posterior shadows, echogenic areas, and hyperechoic rim. Leiomyosarcomas were larger, had more cystic areas, and were associated with a higher prevalence of pelvic free fluid. Smooth muscle tumors of uncertain malignant potential were characterized by a higher frequency of International Federation of Gynecology and Obstetrics (FIGO) type 6-7, the absence of internal shadows, and, in the case of cystic area, the presence of a regular internal wall. Tumor outline varied among the three histological types. A color score of 1 was typical of leiomyoma, a color score 2 was mainly observed in leiomyomas and smooth muscle tumors of uncertain malignant potential, a color score 3 did not differ among the tumors, while a color of score 4 was related to leiomyosarcomas. When combining color scores 3 and 4, leiomyosarcomas and smooth muscle tumors of uncertain malignant potential showed a high percentage of both circumferential and intra-lesional vascularization. A cooked appearance was not statistically different among the tumors. CONCLUSIONS: Based on our findings, specific ultrasonographic features as well as age and menopausal status are associated with different uterine smooth muscle tumor types. Integration of these data can help the pre-operative assessment of these lesions for proper management.

LIN28B regulates transcription and potentiates MYCN-induced neuroblastoma through binding to ZNF143 at target gene promotors.

LIN28B is highly expressed in neuroblastoma and promotes tumorigenesis, at least, in part, through inhibition of let-7 microRNA biogenesis. Here, we report that overexpression of either wild-type (WT) LIN28B or a LIN28B mutant that is unable to inhibit let-7 processing increases the penetrance of MYCN-induced neuroblastoma, potentiates the invasion and migration of transformed sympathetic neuroblasts, and drives distant metastases in vivo. Genome-wide chromatin immunoprecipitation coupled with massively parallel DNA sequencing (ChIP-seq) and coimmunoprecipitation experiments show that LIN28B binds active gene promoters in neuroblastoma cells through protein-protein interaction with the sequence-specific zinc-finger transcription factor ZNF143 and activates the expression of downstream targets, including transcription factors forming the adrenergic core regulatory circuitry that controls the malignant cell state in neuroblastoma as well as GSK3B and L1CAM that are involved in neuronal cell adhesion and migration. These findings reveal an unexpected let-7-independent function of LIN28B in transcriptional regulation during neuroblastoma pathogenesis.

Clinical and Histopathological Predictors of Recurrence in Uterine Smooth Muscle Tumor of Uncertain Malignant Potential (STUMP): A Multicenter Retrospective Cohort Study of Tertiary Centers.

BACKGROUND: The term uterine smooth muscle tumor of uncertain malignant potential (STUMP) indicates a rare, equivocal entity between benign leiomyomas and leiomyosarcomas. In the present study, we evaluated a comprehensive range of clinical, surgical, and pathological features in a large multicenter series of patients with STUMP to identify risk factors for recurrence. METHODS: This is a retrospective study performed by collecting consecutive cases diagnosed between January 2000 and December 2020 in five tertiary centers. Associations between STUMP recurrence and clinicopathological characteristics as well as surgical treatment modality were investigated. RESULTS: Eighty-seven patients affected by STUMP were considered. Of them, 18 cases (20.7%) recurred: 11 as leiomyosarcoma (LMS) and 7 as STUMP. The mean time to recurrence was 79 months. We found that fragmentation/morcellation, epithelioid features, high mitotic count, Ki-67 value > 20%, progesterone receptor (PR) < 83%, and p16 diffuse expression were associated with higher risk of recurrence and shorter recurrence-free survival (RFS). Furthermore, morcellation/fragmentation and mitotic count remained independent risk factors for recurrence and shorter RFS after multivariate analysis, while the presence of epithelioid features was an independent risk factor for recurrence only. CONCLUSIONS: Our results suggest that morcellation is associated with risk of recurrence and shorter RFS, thus it should be avoided if a STUMP is suspected preoperatively. Epithelioid features, high proliferation activity, low PR expression, and diffuse p16 expression are also unfavorable prognostic factors, so patients presenting these features should be closely followed up.

Progesterone Receptor Modulates Extraembryonic Mesoderm and Cardiac Progenitor Specification during Mouse Gastrulation.

Progesterone treatment is commonly employed to promote and support pregnancy. While maternal tissues are the main progesterone targets in humans and mice, its receptor (PGR) is expressed in the murine embryo, questioning its function during embryonic development. Progesterone has been previously associated with murine blastocyst development. Whether it contributes to lineage specification is largely unknown. Gastrulation initiates lineage specification and generation of the progenitors contributing to all organs. Cells passing through the primitive streak (PS) will give rise to the mesoderm and endoderm. Cells emerging posteriorly will form the extraembryonic mesodermal tissues supporting embryonic growth. Cells arising anteriorly will contribute to the embryonic heart in two sets of distinct progenitors, first (FHF) and second heart field (SHF). We found that PGR is expressed in a posterior-anterior gradient in the PS of gastrulating embryos. We established in vitro differentiation systems inducing posterior (extraembryonic) and anterior (cardiac) mesoderm to unravel PGR function. We discovered that PGR specifically modulates extraembryonic and cardiac mesoderm. Overexpression experiments revealed that PGR safeguards cardiac differentiation, blocking premature SHF progenitor specification and sustaining the FHF progenitor pool. This role of PGR in heart development indicates that progesterone administration should be closely monitored in potential early-pregnancy patients undergoing infertility treatment.

Estradiol/nomegestrol acetate as a first-line and rescue therapy for the treatment of ovarian and deep infiltrating endometriosis.

PURPOSE: Estradiol valerate/nomegestrol acetate (E2V/NOMAC) is a new combined oral contraceptive with a good tolerability profile and low drop-out rates, which was shown to improve menstrual-related symptoms. This study aims to evaluate its effectiveness in the control of symptoms and progression of disease in women with ovarian endomestriomas and deep infiltrating endometriosis (DIE). METHODS: This was a retrospective cohort study on 39 women with pelvic endometriosis treated with E2V/NOMAC. We assessed for each patient, at the beginning of treatment and after 6 months, the painful symptoms, through a global VAS (Visual Analogue Scale) index and the size of the greatest ovarian and/or deep infiltrating endometriotic lesions. RESULTS: After 6 months of treatment, a significant reduction was observed for the global VAS score for pain symptoms and for the mean size of ovarian endometriomas, whereas DIE lesions did not present significant changes in mean size. CONCLUSIONS: E2/NOMAC was effective in reducing pain symptoms associated with pelvic endometriosis and the size of ovarian endometriomas, whereas DIE lesions remained stable. This therapy could provide good results in the control of symptoms and disease progression in women with pelvic endometriosis.

Rare case of uterine cervix lymphoma with spontaneous regression: Case report.

Primary malignant lymphoma rarely occurs in the female reproductive tract, because of that they are often misdiagnosed. Lymphoma spontaneous regression is even rarer, but it is possible behavior of this disease. A case of 54-year-old female patient with a primary diffuse large B-cell lymphoma of the cervix is presented. First assumption was sarcoma or atypical adenocarcinoma; biopsies have been inconclusive and, after a partial spontaneous regression, diagnosis of lymphoma was possible only after surgery. The diagnosis was a real challenge for clinicians, radiologists and pathologists for both localization and behavior. Difficulties in diagnosis led to an over-treatment: a laparotomic bilateral hysteron salpingectomy with lymphadenectomy was performed, while chemotherapy alone would have been the right approach. Considering that prognosis and treatment of primary malignant lymphoma of the cervix are completely different than those of other malignant tumors of the uterus, this disease should be considered in the differential diagnosis.

Endometrial thickness in the evaluation of clinical response to medical treatment for deep infiltrating endometriosis: a retrospective study.

PURPOSE: Deep infiltrating endometriosis (DIE) is associated with severe pelvic pain and functional impairment of bowel, urinary, and sexual functions. Though hormone therapy with progestins, either as single agents or combined with estrogens, is effective in managing symptoms, some patients may experience a suboptimal response. Endometrial thickness assessed by transvaginal ultrasound examination, reflecting the overall estrogen stimulation, may correlate with the clinical response to hormonal treatments. METHODS: A retrospective study was carried out on 61 women with DIE affecting the bowel or the recto-vaginal septum, undergoing hormone treatment. The symptoms of patients were evaluated at the baseline and after 12 months of therapy, calculating a global Visual Analogue Scale score (gVAS) encompassing dysmenorrhea, dyspareunia, chronic pelvic pain, dyschezia, abdominal pain and dysuria. Patients were divided into two subgroups using, as a calculated cut-off value, the mean endometrial thickness in our population at 12 months. The change in gVAS score during the 12 months of treatment was then compared between the two groups. RESULTS: Women with a thinner endometrium (< 3.3 mm) showed a better response to treatment in terms of symptoms control as compared to patients with a thicker endometrium (mean gVAS score reduction 9.2 ± 1.3 vs. 5.2 ± 1.3, p = 0.036). The correlation between endometrial thickness and symptomatic response was also confirmed (p = 0.041) on multivariate linear regression analysis including as covariates age, size of lesions of DIE, presence of uterine adenomyosis, ovarian endometriosis and type of medical treatment. CONCLUSION: Endometrial thickness on ultrasound transvaginal examination is correlated with better response rates to hormone therapy in terms of symptoms control. A thinner endometrium, probably resulting from a more efficient suppression of estrogen stimulation, is associated with improved symptoms. These results may aid clinicians in monitoring and tailoring hormonal treatments during follow-up of women with symptomatic DIE.

Reciprocal insulation analysis of Hi-C data shows that TADs represent a functionally but not structurally privileged scale in the hierarchical folding of chromosomes.

Understanding how regulatory sequences interact in the context of chromosomal architecture is a central challenge in biology. Chromosome conformation capture revealed that mammalian chromosomes possess a rich hierarchy of structural layers, from multi-megabase compartments to sub-megabase topologically associating domains (TADs) and sub-TAD contact domains. TADs appear to act as regulatory microenvironments by constraining and segregating regulatory interactions across discrete chromosomal regions. However, it is unclear whether other (or all) folding layers share similar properties, or rather TADs constitute a privileged folding scale with maximal impact on the organization of regulatory interactions. Here, we present a novel algorithm named CaTCH that identifies hierarchical trees of chromosomal domains in Hi-C maps, stratified through their reciprocal physical insulation, which is a single and biologically relevant parameter. By applying CaTCH to published Hi-C data sets, we show that previously reported folding layers appear at different insulation levels. We demonstrate that although no structurally privileged folding level exists, TADs emerge as a functionally privileged scale defined by maximal boundary enrichment in CTCF and maximal cell-type conservation. By measuring transcriptional output in embryonic stem cells and neural precursor cells, we show that the likelihood that genes in a domain are coregulated during differentiation is also maximized at the scale of TADs. Finally, we observe that regulatory sequences occur at genomic locations corresponding to optimized mutual interactions at the same scale. Our analysis suggests that the architectural functionality of TADs arises from the interplay between their ability to partition interactions and the specific genomic position of regulatory sequences.

JMJD-1.2/PHF8 controls axon guidance by regulating Hedgehog-like signaling.

Components of the KDM7 family of histone demethylases are implicated in neuronal development and one member, PHF8, is often found to be mutated in cases of X-linked mental retardation. However, how PHF8 regulates neurodevelopmental processes and contributes to the disease is still largely unknown. Here, we show that the catalytic activity of a PHF8 homolog in Caenorhabditis elegans, JMJD-1.2, is required non-cell-autonomously for proper axon guidance. Loss of JMJD-1.2 dysregulates transcription of the Hedgehog-related genes wrt-8 and grl-16, the overexpression of which is sufficient to induce the axonal defects. Deficiency of either wrt-8 or grl-16, or reduced expression of homologs of genes promoting Hedgehog signaling, restores correct axon guidance in jmjd-1.2 mutants. Genetic and overexpression data indicate that Hedgehog-related genes act on axon guidance through actin remodelers. Thus, our study highlights a novel function of jmjd-1.2 in axon guidance that might be relevant for the onset of X-linked mental retardation and provides compelling evidence of a conserved function of the Hedgehog pathway in C. elegans axon migration.

Predictors of response after a second attempt of pharmacological labor induction: a retrospective study.

PURPOSE: The aim of our study was to assess the outcomes of a prolonged induction carried out with a second sequential cycle of pharmacological stimulation after unsatisfactory response to a first attempt, and to highlight variables correlated with higher response rates. METHODS: A retrospective study was carried out on 157 women who underwent a two-step labor induction by vaginal prostaglandins followed by a second cycle of prostaglandins or intravenous oxytocin. Outcomes assessed were mode of delivery and maternal and neonatal morbidity. Main variables of pregnancy and delivery were collected to identify factors predicting the mode of delivery. RESULTS: Among 157 patients, 63 (40.1%) achieved a vaginal delivery, whereas 94 (59.9%) underwent Cesarean section, 9 women (5.7%) had postpartum hemorrhage; in 2 cases (1.3%), an Apgar score < 7 at 5 min from birth was reported. Higher risk of Cesarean section was observed with advanced maternal age (OR 1.13 for additional year, CI 1.04-1.22) and nulliparity (OR 8.84, CI 2.69-29.06), whereas the response rates were better in carriers of group B streptococcus colonization (OR 0.38, CI 0.17-0.84) and in women with favorable cervical status after the first stimulation (OR 0.81 for additional point of Bishop score, CI 0.70-0.94). CONCLUSION: Labor induction with two cycles of pharmacological stimulation is a procedure with fairly good success rates and a low risk of maternal and neonatal complications. Factors predicting its success encompass younger age, parity, a positive recto-vaginal swab for group B streptococcus and a favorable cervix following the first cycle of stimulation.

The H3K4me3/2 histone demethylase RBR-2 controls axon guidance by repressing the actin-remodeling gene wsp-1.

The dynamic regulation of histone modifications is important for modulating transcriptional programs during development. Aberrant H3K4 methylation is associated with neurological disorders, but how the levels and the recognition of this modification affect specific neuronal processes is unclear. Here, we show that RBR-2, the sole homolog of the KDM5 family of H3K4me3/2 demethylases in Caenorhabditis elegans, ensures correct axon guidance by controlling the expression of the actin regulator wsp-1. Loss of rbr-2 results in increased levels of H3K4me3 at the transcriptional start site of wsp-1, with concomitant higher wsp-1 expression responsible for defective axon guidance. In agreement, overexpression of WSP-1 mimics rbr-2 loss, and its depletion restores normal axon guidance in rbr-2 mutants. NURF-1, an H3K4me3-binding protein and member of the chromatin-remodeling complex NURF, is required for promoting aberrant wsp-1 transcription in rbr-2 mutants and its ablation restores wild-type expression of wsp-1 and axon guidance. Thus, our results establish a precise role for epigenetic regulation in neuronal development by demonstrating a functional link between RBR-2 activity, H3K4me3 levels, the NURF complex and the expression of WSP-1.

Impaired removal of H3K4 methylation affects cell fate determination and gene transcription.

Methylation of histone 3 lysine 4 (H3K4) is largely associated with promoters and enhancers of actively transcribed genes and is finely regulated during development by the action of histone methyltransferases and demethylases. H3K4me3 demethylases of the KDM5 family have been previously implicated in development, but how the regulation of H3K4me3 level controls developmental processes is not fully established. Here, we show that the H3K4 demethylase RBR-2, the unique member of the KDM5 family in C. elegans, acts cell-autonomously and in a catalytic-dependent manner to control vulva precursor cells fate acquisition, by promoting the LIN-12/Notch pathway. Using genome-wide approaches, we show that RBR-2 reduces the H3K4me3 level at transcription start sites (TSSs) and in regions upstream of the TSSs, and acts both as a transcription repressor and activator. Analysis of the lin-11 genetic locus, a direct RBR-2 target gene required for vulva precursor cell fate acquisition, shows that RBR-2 controls the epigenetic signature of the lin-11 vulva-specific enhancer and lin-11 expression, providing in vivo evidence that RBR-2 can positively regulate transcription and cell fate acquisition by controlling enhancer activity.

Sequential polarization and imprinting of type 1 T helper lymphocytes by interferon-gamma and interleukin-12.

Differentiation of naive T lymphocytes into type I T helper (Th1) cells requires interferon-gamma and interleukin-12. It is puzzling that interferon-gamma induces the Th1 transcription factor T-bet, whereas interleukin-12 mediates Th1 cell lineage differentiation. We use mathematical modeling to analyze the expression kinetics of T-bet, interferon-gamma, and the IL-12 receptor beta2 chain (IL-12Rbeta2) during Th1 cell differentiation, in the presence or absence of interleukin-12 or interferon-gamma signaling. We show that interferon-gamma induced initial T-bet expression, whereas IL-12Rbeta2 was repressed by T cell receptor (TCR) signaling. The termination of TCR signaling permitted upregulation of IL-12Rbeta2 by T-bet and interleukin-12 signaling that maintained T-bet expression. This late expression of T-bet, accompanied by the upregulation of the transcription factors Runx3 and Hlx, was required to imprint the Th cell for interferon-gamma re-expression. Thus initial polarization and subsequent imprinting of Th1 cells are mediated by interlinked, sequentially acting positive feedback loops of TCR-interferon-gamma-Stat1-T-bet and interleukin-12-Stat4-T-bet signaling.

H3K23me2 is a new heterochromatic mark in Caenorhabditis elegans.

Genome-wide analyses in Caenorhabditis elegans show that post-translational modifications (PTMs) of histones are evolutionary conserved and distributed along functionally distinct genomic domains. However, a global profile of PTMs and their co-occurrence on the same histone tail has not been described in this organism. We used mass spectrometry based middle-down proteomics to analyze histone H3 N-terminal tails from C. elegans embryos for the presence, the relative abundance and the potential cross-talk of co-existing PTMs. This analysis highlighted that the lysine 23 of histone H3 (H3K23) is extensively modified by methylation and that tri-methylated H3K9 (H3K9me3) is exclusively detected on histone tails with di-methylated H3K23 (H3K23me2). Chromatin immunoprecipitation approaches revealed a positive correlation between H3K23me2 and repressive marks. By immunofluorescence analyses, H3K23me2 appears differentially regulated in germ and somatic cells, in part by the action of the histone demethylase JMJD-1.2. H3K23me2 is enriched in heterochromatic regions, localizing in H3K9me3 and heterochromatin protein like-1 (HPL-1)-positive foci. Biochemical analyses indicated that HPL-1 binds to H3K23me2 and interacts with a conserved CoREST repressive complex. Thus, our study suggests that H3K23me2 defines repressive domains and contributes to organizing the genome in distinct heterochromatic regions during embryogenesis.