The present invention relates generally to the field of human genetics, and more specifically to the detection of a specific type of germline mutations in the BRCA1 gene, which will predispose to breast and ovarian cancer. In addition, the invention relates to the molecular genetic mechanism that may have mediated the genesis of these mutations, in particular the role of Alu repetitive DNA elements present in the intronic regions of BRCA1. The invention further relates to somatic mutations of this type in the BRCA1 gene in human breast and ovarian cancer, and their use in the diagnosis and prognosis of human breast and ovarian cancer. The invention more particularly relates to the screening of this type of BRCA1 mutations in human genomic DNA, which are useful for the diagnosis of inherited predisposition to breast and ovarian cancer.