Auxiliary component of the CatSper complex, a complex involved in sperm cell hyperactivation. pH-dependent Ca(2+) sensor required to activate the CatSper channel. Sperm cell hyperactivation is needed for sperm motility which is essential late in the preparation of sperm for fertilization. Associates with the CatSper complex via direct interaction with CATSPERZ, and senses intracellular Ca(2+). Together with CATSPERZ, associates with the CatSper channel pore and is required for the two-row structure of each single CatSper channel. . This is the function of ENSG00000214360 (EFCAB9, EF-hand calcium binding domain 9).