CYP2S1 is a recently discovered cytochrome P450 enzyme on the basis of homology searches of databases and was found to be homologous to the CYP2 family of enzymes that are known to metabolize xenobiotics (Rylander et al. 2001). CYP2S1 is expressed in skin cells and is inducible by UV radiation, coal tar and all-trans-retinoic acid (atRA), the latter also serving as a substrate for the enzyme. Expression of CYP2S1 was significantly higher in psoriatic plaque than in normal skin. Psoriasis is a chronic hyperproliferative and inflammatory disorder.