For the reference group of dMMR Lynch syndrome tumors, a somatic second hit was identified in 100% of MLH1 (n = 4), in 70% of MSH2 (n = 10), in 100% of MSH6 (n = 1) and 100% of PMS2 (n = 1) germline pathogenic variant carriers, but somatic MMR mutations in the other MMR genes were rarely observed (Table S1). Here, MSH6 is linked to Lynch syndrome.