Background: MSI is a tumor biomarker for immunotherapy efficacy that is closely associated w/deficient MMR, and is also variably associated w/somatic muts of the MMR genes, loss of MMR protein expression by immunohistochemistry (IHC), and high TMB. Germ-line muts (Lynch syndrome/LS) in the 4 MMR genes are recognized to have distinct cancer spectrums/penetrance in LS, yet across all tumors, gene-specific variability in MMR IHC, MMR muts, and TMB is understudied. Methods: Results of TGPs performed by Caris Lifesciences (2015-17) were analyzed in colorectal (CRC), endometrial (EC) and all other cancers/tumors (OT) as: 1) all MSI-H tumors (n = 1057) and 2) all tumors w/ ≥1 mut in an MMR gene (n = 470). A subset of cases had IHC for MMR protein and PD-L1 expression. MSI and TMB were determined by NGS. Results: Characteristics of MSI-H tumors and tumors w/ ≥1 MMR mut are seen in the Table. A single MSH6 mut [F1088fs] in a coding microsatellite represented 31% of all MMR muts detected. F1088fs was found in 58% tumors w/MLH1/PMS2 loss (IHC) but only 25% w/MSH2/MSH6 loss (p < 0.001), was more commonly seen in EC vs CRC/OT (p < 0.001), and was negatively associated w/somatic POLE mut (p = 0.002). Distinct mut signatures in the MMR genes (e.g. GLUàSTOP) were seen in tumors w/POLE muts and DNA repair genes. Conclusions: MSI-H and MMR mutated tumors demonstrate marked gene-specific heterogeneity in IHC patterns, TMBs, and somatic muts that may be relevant to treatment selection, resistance, and response.