LBA1509 Background: The success of immunotherapy in microsatellite unstable (MSI-H) and/or DNA mismatch repair deficient (MMR-D) tumors has resulted in routine MSI-H/MMR-D testing in advanced solid tumors. Unlike colorectal (CRC) and endometrial cancer (EC), where this has long been undertaken, the characterization of Lynch syndrome (LS) across heterogeneous MSI-H/MMR-D tumors is unknown. Methods: Through a targeted NGS panel, MSI status was determined via MSIsensor. Scores of < 3, ≥3 to < 10, or ≥10 designated Microsatellite stable (MSS), MSI-Indeterminate (MSI-I) or MSI-H status, respectively. Germline mutations were assessed in MLH1, MSH2, MSH6, PMS2, EPCAM. Immunohistochemical staining (IHC) for MMR-D and tumor signatures in LS patients were assessed. Clinical variables were correlated with MSI and compared via Chi square or T-test. Results: Of 15,045 tumors spanning > 50 cancers , 93.2% were MSS, 4.6% MSI-I, and 2.2% MSI-H. Germline mutations were identified in 0.3% (37/14,020), 1.9% (13/699), and 16.3% (53/326) in the MSS, MSI-I, and MSI-H groups, respectively (p-value < 0.001). 25% of 1,025 MSI-H/MSI-I tumors were CRC/EC, but 50% (33/66) of LS patients had MSI-H/MSI-I tumors less commonly or not previously associated with LS (mesothelioma, sarcoma, adrenocortical, melanoma, ovarian germ cell). LS pts with MSI-H/MSI-I non-CRC/EC tumors only met testing criteria in 63.6% of cases, had lower MSIsensor scores, and were more likely to be MSI-I (MSI-I: non-CRC/EC, 30.3% (10/33) vs CRC/EC 9.1% (3/33); p-value = 0.03). IHC was completed in 86.4% (57/66) of LS MSI-H/MSI-I tumors, with 98.3% MMR-D-concordance. Of LS pts with MSS tumors, 78% had MSH6/PMS2 mutations, but 71% of LS pts with MSI-H/MSI-I tumors had MLH1/MSH2/EPCAM mutations(p-value < 0.001). 89% (33/37) of MSS tumors of LS pts had non-MMR-D signatures. Conclusions: MSI-H/MMR-D is predictive of LS across tumor types and suggests a more heterogeneous spectrum of LS-associated cancers than previously appreciated. Nearly 40% of LS pts with MSI-H/MMR-D non-CRC/EC tumors did not meet clinical criteria for genetic testing, suggesting that MSI-H/MMR-D tumors, regardless of cancer type or family history, should prompt germline testing for the evaluation of LS.