MRC1 and cancer: Therefore, although somatic MMR deficient cancers are usually microsatellite-unstable, ultra-hypermutated GBMs from CMMRD (or from Lynch syndrome in this case) with MMR first/POLE second are predominantly microsatellite-stable, similar to the ultra-hypermutated colorectal and endometrial cancers with somatic MMR and POLE proofreading deficiency (Cancer Genome Atlas 2012; Cancer Genome Atlas Research et al. 2013).