BRCA1 mutation carriers are at increased risk for both breast and ovarian cancers. The Consortium of Investigators of Modifiers of BRCA1/2 (CIMBA) have previously reported a two-stage genome-wide association study (GWAS) for modifiers of breast or ovarian cancer risk in BRCA1 mutation carriers (Nat. Genet. 42, 885–892, 2010), identifying a 19p13 locus associated with breast cancer risk for individuals with BRCA1 mutation. Fergus Couch and colleagues now report a large-scale replication study including 11,705 BRCA1 mutation carriers from 45 centers in 25 countries (PLoS Genet. 9, e1003212, 2013). These samples were all genotyped using the custom iCOGS array, which included 32,557 SNPs selected from the original GWAS. The authors identified a new susceptibility locus at 1q32, containing the MDM4 gene, associated with breast cancer risk for BRCA1 mutation carriers and also identified two new loci associated with ovarian cancer risk for BRCA1 mutation carriers at 17q21.31 and 4q32.2, with the latter representing the first locus to modify cancer risk specifically in BRCA1 mutation carriers. The authors also replicated previous associations and report a total of ten and seven loci associated, respectively, with breast and ovarian cancer risk in BRCA1 mutation carriers. On the basis of combined-SNP profiles, they report large differences in the predicted risk of developing breast or ovarian cancer for the 5% of BRCA1 mutation carriers at highest and lowest risk.