Global Chromatin Reprogramming in Fetal Germ Cells Figure 7. Global changes of H3K79me2 in fetal germ cells. Gonad preparations were stained with anti-H3K79me2 antibody. DAPI was used to visualize nuclei (blue). Other details are as in legend to Figure 1. doi:10.1371/journal.pone.0023848.g007 levels of globular histone marks H3K79me3 and H3K79me2 strongly increased at 15.5 dpc. H3K79me3 and H3K79me2 occupy reciprocal, methylated versus unmethylated parental alleles at DMRs [38] and generally cluster with repressive and active histone marks, respectively in chromosome-wide mapping [40]. They also globally occupy different nuclear regions, H3K79me3 is localized to DAPI-stained heterochromatin whereas H3K79me2 is localized to DAPI-poor regions (Figure 9) and [39]. It is possible that newly formed chromatin patterns (for example H3K79me3-H3K9me3 versus H3K79me2-H3K4me3-rich regions) target and repel de novo DNA methylation at specific genomic regions, respectively and thus help establish the prospermatogonia-specific DNA meth- ylation pattern. Our recent study provided the first information on chromatin at a specific locus in gestational age germ cells. We showed that at the H19/Igf2 gametic DMR, the erasure of allele-specific chromatin follows, and therefore, is not required for CpG demethylation in PGCs [24]. No data exists, however, on chromatin at single loci during the DNA remethylation stage. It will be important to perform chromatin mapping in fetal germ PLoS ONE | www.plosone.org 8 August 2011 | Volume 6 | Issue 8 | e23848