BRCA1 and breast cancer: In the literature, cases of BC arising in twins have been reported, most commonly associated with BRCA1/2 variants.8, –10 The incidence of BC in monozygotic twins is four times higher than in the mothers and sisters of patients, especially after age 55, with a lifetime cumulative risk of BC for identical twins estimated at 20%–30%.11 A simple Mendelian genetic model, however, does not fully account for this elevated risk, as it predicts much lower rates of BC in this population.