MRC1 and mismatch repair cancer syndrome 1: Although screening for MMR deficiency in breast tumors is currently not standard clinical practice, multiple studies have shown that breast cancers from women with LS are more likely to exhibit microsatellite instability and loss of one or more MMR proteins via immunohistochemistry, compared with sporadic breast cancers.3,4,5,6,7,8,9,10,11,12,13,14