Among individuals of AJ ancestry, most breast cancers are due to three BRCA1/2 founder mutations (BRCA1: 185delAG [c.68_69del], 5382insC [c.5266dup]; BRCA2: 6174delT [c.5946del]); while in the Mexican population the founder mutation BRCA1 ex9-12del has a high frequency. Here, BRCA1 is linked to breast cancer.