Spef is a region of sperm flagellar proteins. It probably exerts a role in spermatogenesis in that the protein is expressed predominantly in adult tissue. It is present in the tails of developing and epididymal sperm internal to the fibrous sheath and around the dense outer fibers of the sperm flagellum. The amino-terminal domain (residues 1-110) shows a possible calponin homology (CH) domain; however Spef does not bind actin directly under in vitro conditions, so the function of the amino-terminal calponin-like domain is unclear. Transcription aberrations leading to a truncated protein result in immotile sperm.