B2M and neoplasm: As in previous studies, we found that B2M mutations are rare in pMMR CRC (< 3%), as it is the coding microsatellites in the B2M gene which are most vulnerable to mutation in the presence of dMMR.13, 15, 17 Koelzer and colleagues20 found a somewhat higher rate of B2M protein loss among pMMR tumours (22 of 310; 7%), although three (14%) of these 22 tumours recurred and no survival advantage was demonstrated.