As a result, many study cohorts consisted only of MLH1 and MSH2 carriers although some also included MSH6. 17,18,19,20,21,22,23 A few studies have included all four MMR genes, but presented only a single combined breast cancer risk, likely due to a small number of MSH6 and PMS2 carriers.15,16 Thus, the lack of consistent association of breast cancer and LS in previous studies may reflect the genetic composition of the cohorts. Here, MRC1 is linked to breast cancer.