CYP51 is the only gene of the cytochrome P450 (P450, or CYP) superfamily that is expressed in prokaryotes and eukaryotes. In animals, the gene product, P45014DM, catalyzes the lanosterol 14alpha-demethylase reaction, an essential step in cholesterol biosynthesis. P45014DM serves a housekeeping role, and it was surprising to find the highest level of CYP51 expression in the testes. This is a result of very high-level CYP51 expression in postmeiotic, haploid spermatids and results in elevated P45014DM activity in these cells. It is proposed that the elevated level of 14alpha-demethylase activity leads to production of signaling sterols by haploid germ cells, although the function of such sterols in males is unknown.