CYP1B1 and breast cancer: The valine substitution is reported to be present at an allele frequency of 0.43 in Caucasians, 0.75 in African-Americans and 0.23 in Asians.12,25 No association between the CYP1B1 Val432Leu polymorphism and breast cancer was observed in Asians (for Val/Val and Val/Leu combined, OR = 1.0, 95% CI: 0.8, 1.2).