The cytochrome P450 proteins are monooxygenases that catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids and other lipids. The CYP2C (CYPIIC) subfamily comprises a group of constitutively expressed genes, some phenobarbital-induced genes, and several genes associated with sex-specific expression. Originally, at least 7 genes were assigned to the CYP2C subfamily, which maps to human 10q24.1-q24.3 and mouse chromosome 19. Humans contain fewer genes in this cluster than do mice or rats. Four genes are arranged in the following order and orientation along human chromosome 10: RBP4, CYP2C18, CYP2C19/CYP2C9, and CYP2C8. The previsously identified CYP2C10 is an artifact of CYP2C9 and not a distinct isoform. All of the CYP2C proteins localize to the endoplasmic reticulum.