This family of proteins found in eukaryotes represents sperm acrosome-associated protein 9 (SPACA9, previously known as C9orf9 or MAST).Sperm acrosome-associated protein 9 has been suggested to form a complex with calcium-binding proteins calreticulin and caldendrin localized to the acrosome. Despite this, no known protein interaction motifs have been identified in MAST [].