The increased risk of the double heterozygotes (P187S/R72P) to develop breast cancer is a new finding, which is similar to the observation that double heterozygotes for the Factor V “Leiden” and the Prothrombin mutation G20210>A have a 20-fold risk for developing thrombosis, whereas the risk for single heterozygotes is only five-fold and four-fold, respectively (Emmerich et al, 2001), and agrees with the two-locus genetic model of Risch (2000). The gene discussed is F2; the disease is breast cancer.