Thus, while case-control studies suggested the SNP34091C-allele to be associated with a reduced risk for breast cancer in general [17, 21] and among individuals carrying a MDM2 SNP309GG genotype in particular [17], recent genome-wide association studies (GWAS) have reported the C-allele to be associated with an increased risk for estrogen receptor (ER) negative and, in particular, triple-negative breast cancer [22–24]. Here, ESR1 is linked to breast cancer.