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 (By similarity). Monomer. Testis-specific. Expressed mainly in round spermatids. First expressed after puberty. Contains 3 inactive NDK domains that each lack the active His residue, suggesting that they have no NDP kinase activity. In the C-terminal section; belongs to the NDK family.