CatSpers are ion transport proteins located on the surface of sperm cells in the principal piece of the sperm tail. CatSpers are vital to sperm motility, fertilization and cAMP-mediated calcium influx in sperm. There are four CatSper proteins in mammalian sperm, namely CatSper (or CatSper1), CatSper2, CatSper3 and CatSper4. CatSper proteins are believed to assemble into a heterotetrameric complex, forming an alkalinization-activated Ca2+-selective channel. CatSper∫ is a multipass membrane protein that functions as an auxiliary protein of the CatSper channel. It contains a large extracellular domain and a number of N-glycosylation sites. CatSper∫ specifically interacts with CatSper1 and through this interaction plays an important role in the hyperactivated motility of sperm cells, a process that is required in the preparation of sperm for fertilization.