Although the risk to a woman from a Jewish multiplex family (Egan et al, 1996) or a Jewish family with a BRCA1/2 mutation (Fodor et al, 1998) is increased by a factor of 3–4, and although Jewish women are also at higher risk from additional breast cancer risk factors (Swift et al, 1987), we found that a Jewish MZ co-twin's risk of becoming affected was increased only marginally more than that to Jewish women generally (Mack et al, 1985; Warner et al, 1999). Here, BRCA1 is linked to breast cancer.