**2.9 CYP26A1 (cytochrome P450 family 26 subfamily A member 1)**

CYP26A1 is a cytochrome P450 monooxygenase that plays a key role in the metabolism of ATRA. The cytochrome P450 superfamily contains 57 members that are monooxygenase enzymes which catalyze many processes including drug metabolism and synthesis of cholesterol, steroids, and various lipids. CYP26A1 acts on ATRA by catalyzing the hydroxylation of carbon hydrogen bonds of ATRA. This includes both 4-hydroxylation and 18-hydroxylation activities. It has little activity toward 9-cis and 13-cis RA ligands. By regulating intracellular concentrations of RA, CYP26A1 can control RA signaling mediated gene expression in both embryonic and adult tissues. There are two alternatively spliced transcript variants of CYP26A1 that encode the different isoforms. This enzyme regulates the cellular level of RA which in turn regulates gene expression in both embryonic and adult tissues. Diseases associated with CYP26A1 include embryonal carcinoma and APL.
