Exclusion of the inactive chromatin mark histone H3K27me3 from unsynapsed chromosomal regions is limited to the XY-bivalent in single translocation carriers.

Z - zygotene, EP – early pachytene, LP – late pachytene, D – diplotene spermatocytes. Arrows point to the XY bivalents. Arrowheads indicate unsynapsed trivalents. A – Zygotene spermatocyte is enriched for H3K27me3; B-F - the H3K27me3 histone mark is excluded from the XY-bivalent in early (B) mid (C), late (D, E) pachytene and diplotene (F) spermatocytes. However, H3K27me3 is not excluded from the unsynapsed trivalent (B, C and E). D - Only one of 15 late pachytene spermatocytes with unsynapsed trivalent showed H3K27me3 exclusion.