A 28-year-old male presented with diffusely metastatic epi thelioid-type peritoneal mesothelioma (MPeM)...Next generation sequencing with a 468 gene panel (320X coverage) revealed...tumor muta tional burden (TMB) of 1.8 mutations/megabase, including loss of function pathogenic mutations in BAP1 (V99M: c.295G > A) and NF2 (R34:1c.1021C >T), as well as broad copy number loss on chromosome arms 14q and 19q. Pembrolizumab (200 mg) was then added to the pemetrexed/ carboplatin every 3 weeks (Q3W) for 13 total treatments over 8 months. After pembrolizumab addition, the patient exhibited markedly improved functional status with resumption of normal bowel movements and decreased abdominopelvic ascites, abdomi nal nodularity, and peritoneal thickening.