Plays a key role in the replacement of histones to protamine in the elongating spermatids of mammals (PubMed:11772016). In condensing spermatids, loaded onto the nucleosomes, where it promotes the recruitment and processing of protamines, which are responsible for histone eviction (By similarity). Loaded onto the nucleosomes of condensing spermatids (By similarity). Nuclear import is mediated by IPO4 (PubMed:17682055). Nucleolar localization requires the protein to be phosphorylated (PubMed:14514679). Testis. Expressed during stages 12-15 of spermiogenesis. Belongs to the nuclear transition protein 2 family.