CYP51 is an evolutionarily conserved, housekeeping gene of the cytochrome P450 superfamily which is involved in cholesterol biosynthesis in animals. The two intermediates of cholesterol biosynthetis pathway, sterol FF-MAS, produced by CYP51 and the following sterol T-MAS, accumulate in follicular fluid and in testis. CYP51 is expressed in all tissues in mammals with the highest level in the testis. In rat, expression peaks in postmeiotic male germ cells of the testis. Promoters of the human and rat CYP51 genes contain SRE and CRE elements which indicate two main regulatory routes--the sterol-dependent regulation and the cAMP-dependent regulation. While feedback regulation by sterols is characteristic for all genes involved in cholesterol biosynthesis and homeostasis, the cAMP-dependent regulation is unique, indicating that CYP51 may play tissue-specific roles distinct from cholesterol biosynthesis.