Chromatin functions as a physical barrier regulating access to DNA, however pioneer factors are able to engage partial recognition motifs present within nucleosomal DNA to initiate access to specific DNA sequences. During spermatogenesis, genomic locations that become recombination hotspots are generally in regions of closed chromatin, or heterochromatin, before meiosis. However, in leptotene and zygotene stages of meiosis, PRDM9 marks nucleosomes at recombination hotspots with H3K4me3 and H3K36me3 and recruits other factors that deposit additional histone marks. Here I focus on the technological approaches by which we discovered that hotspots also transition from closed to open chromatin, dependent on the chromatin remodeler HELLS. HELLS and PRDM9 form a pioneer complex that targets hotspot sequences and provides access for the DNA double-strand break (DSB) machinery. Failure to reorganize chromatin at hotspots into an open conformation leads to DSBs at other open regions, such as promotors, causing a breakdown in recombination and cell death
