Nuclear autoantigenic sperm protein is a H1 histone binding protein that is involved in transporting histones into the nucleus of dividing cells. Multiple isoforms are encoded by transcript variants of this gene.

The protein sequence is 53% identical to that of the Xenopus ortholog, and 85% identical to that of the rabbit ortholog when the N terminal of the rabbit sequence is aligned with amino acid 101 of the human sequence. Two histone-binding domains are strongly conserved. Northern blot analysis revealed a 3.2 kb mRNA. Immunolocalization experiments demonstrated that in the testis, NASP is found primarily in the cytoplasm of primary spermatocytes and around or within the nuclei of spermatids. NASP is present near the acrosomal region in mature human sperm.