4144 Background: Microsatellite instability (MSI-H) occurs in the setting of Lynch syndrome (LS), which is caused by germline pathogenic variants (gPV) in mismatch repair (MMR) genes. The incidence and implications of somatic MMR variants without LS gPV in PC is unknown. We present a large cohort of patients (pts) with somatic only MMR variants and PC, and outline clinico-genomic descriptors, MSI-H prevalence, MMR methylation status, and response to ICB, comparing these features with a LS PC cohort. We also present microsatellite status (MS) orthogonal testing strategies. Methods: Institutional databases queried to identify pts with somatic MMR variants or LS gPVs and PC consented to matched tumor-normal NGS from 2012-2023. Clinico-genomic data abstracted from medical record. MiMSI used to assess MS stable (MSS) and indeterminate (MSI-I) PC. MMR status determined by immunohistochemistry (IHC). Composite MSI-H adjudicated as loss of MMR protein expression by IHC or MSI-H interpretation by MSISensor or MiMSI. Zygosity status assessed with FACETs. Exploratory mutational signature analysis performed on tumor mutation burden-high (TMB-H > 10) tumors with deconstructSigs. MMR methylation analyses underway. Results: N=55/4,626 (1.2%) pts with PC and MMR variant: N=23 somatic MMR variant only (Somatic Cohort) and N=32 LS Cohort. In Somatic Cohort, 10 (43%) MSI-H cases; 6 reclassified as MSI-H by MiMSI (N=2 MSI-I; N=4 MSS). Zygosity status: 11 (48%) biallelic, 11 (48%) monoallelic, 1 (4%) indeterminate. Of 11 monoallelic cases, 3 MSI-H. N=7/14 (50%) evaluable pts had dominant MSI-H mutational signatures. In LS Cohort, 17 (59%) MSI-H cases; 5 reclassified as MSI-H by MiMSI (N=4 MSI-I; N=1 MSS). Zygosity status (N=27 evaluable): 9 (33%) biallelic, 15 (56%) monoallelic, 3 (11%) indeterminate. Of 15 monoallelic cases, 4 MSI-H. N=7/15 (44%) evaluable pts had dominant MSI-H signatures. N=20 pts received ICB (N= 14 LS; N= 6 Somatic Cohort). Median ICB duration: 27.7 months (95% confidence interval (CI) 11.5, -). Overall response rate: 50% in both cohorts; similar duration of response. Conclusions: MSI-H arises due to somatic and germline oncogenesis at similar rates in ~1% of PC. MMR methylation analyses may provide further insight into the mechanisms of MSI-H. Orthogonal MS testing key to inform accurate MS status. MiMSI valuable in classifying low-cellularity specimens. Pts with MSI-H PC treated with ICB derive durable benefit, irrespective of germline or somatic etiology. [Table: see text]