MSH6 and breast cancer: 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.