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 fibres of the sperm flagellum [1]. 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 [1]. Transcription aberrations leading to a truncated protein result in immotile sperm [2].

1: Spef1, a conserved novel testis protein found in mouse sperm flagella. Chan SW, Fowler KJ, Choo KH, Kalitsis P; Gene. 2005;353:189-199. PMID:15979255

2: An intronic insertion in KPL2 results in aberrant splicing and causes the immotile short-tail sperm defect in the pig. Sironen A, Thomsen B, Andersson M, Ahola V, Vilkki J; Proc Natl Acad Sci U S A. 2006;103:5006-5011. PMID:16549801