Among the 444 breast cancers analyzed, 41 (9%), 55 (12%), 24 (10%), and 21 (5%) cases showed homogeneous loss of MLH1, MSH2, MSH6, and PMS2, respectively, and the cases showing intra-tumor heterogeneity ranged from 7 (1%) for PMS2 to 42 (10%) for MSH2 (Supplementary Table 3, available online). The gene discussed is MSH6; the disease is neoplasm.