Sterol biosynthesis requires the removal of the 14 alpha-methyl group from lanosterol in animals and fungi and from obtusifoliol in plants. This reaction is catalyzed by a microsomal cytochrome P450, the sterol 14 alpha-demethylase (P450(14DM), which is the only P450 described so far to be expressed in different phyla. A cDNA encoding human P450(14DM) was isolated from a liver cDNA library using a partial rat lanosterol 14 alpha-demethylase cDNA probe. The deduced amino acid sequence is 93% and 38--42% identical to rat and fungal P450(14DM), respectively. Expression of the human CYP51 cDNA in Escherichia coli showed that the cDNA encodes an enzyme having lanosterol 14 alpha-demethylase activity. Northern blot analysis showed that CYP51 mRNA is ubiquitously expressed with highest levels in testis, ovary, adrenal, prostate, liver, kidney, and lung. Many genes involved in cholesterol homeostasis are regulated by cholesterol or its metabolites. In the case of CYP51, cholesterol deprivation led to a 2.6- to 3.8-fold induction of mRNA levels in human adrenocortical H295R cells and this effect was suppressed by the addition of 25-hydroxycholesterol. In human hepatoma HepG2 cells, no effect of cholesterol deprivation was observed; however, the levels of CYP51 mRNA were reduced 4- to 6-fold by the addition of 25-hydroxycholesterol. Thus, like several other genes in the cholesterol biosynthetic pathway, including the 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) synthase, HMG CoA reductase, squalene synthase, and farnesyl diphosphate synthase, the expression of the human CYP51 is suppressed by oxysterols.