May be required for post-translational modifications of proteins required for acrosomal biogenesis. May act by reducing disulfide bonds within the sperm. Testis-specific. Only expressed during spermiogenesis, prominently in the Golgi apparatus of pachytene spermatocytes and round and elongated spermatids, with a transient localization in the developing acrosome of round spermatids (at protein level). Increased levels, possibly caused by overexpression, are observed in morphologically abnormal spermatozoa from infertile men, suggesting that it may be used as a marker of aberrant spermatogenesis. Belongs to the thioredoxin family.