Required for proper cytoplasm removal during spermatogenesis. Testis-specific. Exclusively present in cytoplasm of steps 14-16 elongated spermatids (at protein level). Mice are completely infertile because of deformed sperm characterized by a bent head wrapped around by the neck and the middle piece of the tail. Lack of Spem1 causes failure of the cytoplasm to become loose and detach from the head and the neck region of the developing spermatozoa.