Cytochrome P450 1B1 is a key enzyme involved in the production of potentially carcinogenic estrogen metabolites and the activation of environmental carcinogens and is the predominant member of the CYP1 family expressed in normal breast tissue and breast cancer. Estrogen has been proposed to trigger breast cancer development via an initiating mechanism involving its metabolite, catechol estrogen (CE). CYP1B1 catalyzes the conversion of 17-beta-estradiol to the catechol estrogen metabolites 2-OH-E2 and 4-OH-E2 which have both been postulated to be involved in mammary carcinogenesis. 
Genetic polymorphisms in CYP1B1 may play an important role in human prostate carcinogenesis as well. Polymorphism of the CYP1B1 gene at codon 432 (Val-->Leu) is associated with a change in catalytic function.