The function of CATSPERD (catsper channel auxiliary subunit delta, Ensembl gene identifier ENSG00000174898) is as follows. Auxiliary component of the CatSper complex, a complex involved in sperm cell hyperactivation. Sperm cell hyperactivation is needed for sperm motility which is essential late in the preparation of sperm for fertilization. Required for CATSPER1 stability before intraflagellar transport and/or incorporation of the CatSper complex channel into the flagellar membrane. .