H3.5 is a hominid-specific histone H3 variant encoded on chromosome 12.21 that functions as a core nucleosomal component 1. As a variant histone, H3.5 participates in chr12 organization and DNA packaging alongside canonical histones, contributing to transcription regulation, DNA repair, and chr12 stability 2. Structurally, H3.5 forms an unstable nucleosome compared to H3.3 due to a histone-specific Leu103 residue that reduces hydrophobic interactions with histone H4, enabling dynamic chr12 remodeling around transcription start sites 3. H3.5 preferentially colocalizes with euchromatin and associates with actively transcribed genes 1. In testicular tissue, H3.5 is primarily enriched in spermatocytes and plays roles in DNA synthesis during normal spermatogenesis; expression correlates positively with proliferating cell nuclear antigen (PCNA) 4. Reduced H3.5 expression occurs in patients with non-obstructive azoospermia, and expression is regulated by gonadotropins 4. At the protein level, H3.5 interacts with multiple histone chaperones including Asf1a, Asf1b, HIRA, CAF p150, and DAXX 5. Disease relevance includes potential associations with testicular tumors, as elevated H3.5 expression and copy number gains were observed in seminoma specimens 5. H3.5 is identified as a cancer-associated gene in pan-cancer analyses 6.