Calcium-binding protein (PubMed:19208547). Essential for maintaining the structural integrity of the sperm flagella (PubMed:33440775). Mostly cytoplasmic, but associated with the mitochondrial inner membrane during the last steps of spermatid differentiation (By similarity). Localizes to the principal piece of the sperm flagellum (PubMed:19208547, PubMed:33440775). Detected only in testis. Expressed from stages X to VIII of the seminiferous epithelial cycle. Expressed from step 13 to step 16 of spermatid development (at protein level). Down-regulated by Busulfan. Males exhibit significantly impaired sperm tail structure and subfertility (PubMed:33440775). Defects in sperm flagellar differentiation leads to an abnormal annulus and disorganization of the midpiece-principal piece junction (PubMed:33440775).