ABSTRACTIntroduction. Rare solid tumors account for one-quarter of cancers among adults in the United States, but few resources have been devoted to their treatment. We evaluated the efficacy of pembrolizumab, a programmed cell death-1 inhibitor, in patients with rare solid tumors.Methods. We conducted a phase 2 basket trial that included patients with rare, advanced tumors. Patients were enrolled in the study in nine tumor-specific and a 10th cohort of miscellaneous rare histologies. Patients received pembrolizumab 200 mg intravenously every 21 days. The primary endpoint was the non-progression rate at 27 weeks per immune-related Response Evaluation Criteria in Solid Tumors (RECIST). The secondary endpoints were confirmed objective response (immune-related complete response [irCR] or partial response [irPR]), clinical benefit (irCR, irPR, or immune-related stable disease [irSD] ≥ 4 months), safety, and tolerability. Pretreatment biopsy specimens were examined for programmed cell death