CatSper (cation channel of spermatozoa) is a sperm-specific Ca²⁺ channel located explicitly in the spermatozoon flagellum. It mediates flagellar hyperactivity, and is chemotactically responsive to progesterone. CatSper is composed of 4 pore-forming subunits (CatSper1-4) with 6 transmembrane (TM) domains each, and of 3 auxiliary subunits: CatSperβ (2 TM domains), CatSperγ and CatSperδ (1 TM domain each). While all four CatSper proteins are necessary for the channel’s function (and hence male fertility), each is synthesized independently. Trafficking to the flagellum requires the cooperation of all subunits.