CYP1B1 and breast cancer: The cytochrome P450 1B1 (CYP1B1) is the only member in the CYP1B subfamily and shares 40% homology with two members in the CYP1A subfamily (CYP1A1 and CYP1A2).15CYP1B1 has the highest catalytic activities toward several polycyclic aromatic hydrocarbons that are the most potent inducers of mammary tumors and lung cancers.16 Apart from these exogenous compounds CYP1B1 is also involved in the metabolism of endogeneous steroids such as 17β estradiol.