We recently performed a discovery genome-wide association study (GWAS) using DNA from women who received tamoxifen or raloxifene during the P-1 and P-2 breast cancer prevention trials with a phenotype of breast cancer occurrence during 5 years of SERM preventive therapy.6 That GWAS identified two top single nucleotide polymorphism (SNP) signals that mapped to the second intron of the zinc finger protein 423 (ZNF423) gene on chromosome 16 and 5’ of the cathepsin O (CTSO) gene on chromosome 4. Here, CTSO is linked to breast cancer.