The molecular diagnosis of genetic abnormalities responsible for genetic predisposition to breast cancer is made difficult by the large size of the genes and the diversity of gene mutations found within these genes. The molecular diagnosis of responsible mutations requires the implementation of particular analytical methods, for which we give two examples, the protein truncation test and the direct sequence analysis of the cDNA. Results obtained with these two methods demonstrate the interest of studying the sequence of messenger RNA expressed by predisposing genes. The study also describes an abnormal splicing and two rearrangements responsible for genetic predisposition to breast cancer.