Plays a key role in the replacement of histones to protamine in the elongating spermatids of mammals. In condensing spermatids, loaded onto the nucleosomes, where it promotes the recruitment and processing of protamines, which are responsible for histone eviction. Loaded onto the nucleosomes of condensing spermatids. Expressed by spermatids (at protein level). Belongs to the nuclear transition protein 1 family.