Essential for normal spermatogenesis and male fertility. Required for proper cell restructuring and DNA condensation during the elongation phase of spermiogenesis. Involved in the histone-protamine transition of sperm chromatin and the subsequent production of functional sperm. Binds both double-stranded and single-stranded DNA, ATP and protamine-1. In round and elongating spermatids, specifically localizes to a chromatin domain at the apical pole. Testis-specific. Specifically expressed in haploid germ cells. First detected in round spermatids at stage 4 and expression strongly increases through stages 5-8. Localization in the nucleus is highly polar and it is concentrated in a cap-like structure at the inner periphery of the nuclear membrane. Polarized expression persists in steps 9-14 elongating spermatids before rapidly disappearing by stage 15. Belongs to the histone H1/H5 family.