Probably required during the final stages of sperm tail maturation in the testis and/or epididymis, where extensive disulfide bonding of fibrous sheath (FS) proteins occurs. May be involved in the reduction of disulfide bonds within the sperm FS components. In vitro, it has neither NDP kinase nor reducing activity on disulfide bonds. Monomer. Testis-specific. Expressed only in primary spermatocytes and round spermatids. Restricted to spermiogenesis, starting at the pachytene spermatocyte level and peaking at the round and elongating spermatid stage. Contains 3 inactive NDK domains that each lack the active His residue, suggesting that they have no NDP kinase activity. The disease is caused by variants affecting the gene represented in this entry. In the C-terminal section; belongs to the NDK family.