In order to identify additional association signals at the12p11 locus with breast cancer risk, understand the underlying mechanisms and potential causal variants responsible for the association, we conducted a large fine-scale mapping study including data from 55,540 breast cancer cases and 51,168 controls in the Breast Cancer Association Consortium (BCAC) and 15,252 BRCA1 mutation carriers in the Consortium of Investigators of Modifiers of BRCA1/2 (CIMBA). This evidence concerns the gene BRCA1 and breast cancer.