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. In spermatozoa, it is an integral component of the FS. Testis-specific. Prominently detected in the cytoplasmic lobe of step 15-18 spermatids and diminishes in step 19 just before spermiation. In the spermatid tail, it increases from step 15 to 19 and is confined to the principal piece. First detected scattered throughout the cytoplasm of the axoneme in step 14-15 spermatids, but begins to be incorporated by step 16 into the FS. During steps 17-18, it increases over the ribs and columns of the assembled FS. It peaks in step 19 and remains in the FS of epididymal spermatozoa. Contains 3 inactive NDK domains that each lack the active His residue, suggesting that they have no NDP kinase activity. In vasectomized rats, autoantibodies against Txndc3 are present. In the C-terminal section; belongs to the NDK family. Truncated N-terminus.