B2M and hyperinsulinemic hypoglycemia, familial, 4: The B2M gene contains several coding microsatellites, making it a mutagenic target in the presence of mismatch repair deficiency.12 Somatic B2M mutations are found in approximately 30% of dMMR CRCs, but are rare (<2%) in CRCs with proficient mismatch repair.13, 14, 15, 16, 17 In a consecutive series of MSI‐H CRCs, Kloor and colleagues16 detected B2M mutations in 29 of 104 (28%).