Global Chromatin Reprogramming in Fetal Germ Cells Active histone tail marks Acetylation of residue K9 and trimethylation of residue K4 in the tail of histone H3 are well-known components of active chromatin. Germ cells and somatic cells of the fetal gonad were stained with antibodies against H3K9ac and H3K4me3. Male and female germ cells stained strongly for H3K9ac compared to somatic cells at each developmental stage (Figure 2). Male germ cells reached the highest level of H3K9ac at 15.5 dpc. No peak was observed in female germ cells. H3K4me3 levels were very similar between germ cells and somatic cells in the female throughout fetal development and in the male at 11.5–13.5 dpc (Figure 3). H3K4me3 levels increased in male germ cells at 15.5 and reached even higher levels at 17.5 dpc. Repressive histone tail marks Di-and trimethylation of residue K9 and trimethylation of residue K27 in the tail of histone H3 are well-known components of repressive chromatin. Germ cells and somatic cells of the fetal gonad were stained with antibodies against chromatin components H3K9me2, H3K9me3 and H3K27me3. Fetal germ cells stained Figure 3. Global changes of H3K4me3 in fetal germ cells. Gonad preparations were stained with anti-H3K4me3 antibody. DAPI was used to visualize nuclei (blue). Other details are as in legend to Figure 1. doi:10.1371/journal.pone.0023848.g003 PLoS ONE | www.plosone.org 4 August 2011 | Volume 6 | Issue 8 | e23848