E2 also induced the expression of CTSO, and WT genotypes for SNPs near the CTSO gene with low P values were associated with the E2-dependent induction of CTSO expression and, subsequently, BRCA1 in the presence of estrogen, but this induction did not occur in the presence of variant CTSO SNP genotypes, probably because one of the SNPs (rs6813983) near that gene disrupted an ERE.6 Variant sequences for the CTSO-related SNPs were associated with increased risk for breast cancer. Here, CTSO is linked to breast cancer.