 In this Phase II clinical trial with 47 patients, when cancer patients with a defect in MMR in their tumors were exposed to an inhibitor of PD-1, 67% - 78% of patients experienced immune-related progression-free survival.In contrast, for patients without defective MMR, addition of PD-1 inhibitor generated only 11% of patients with immune-related progression-free survival.Thus inhibition of PD-1 is primarily synthetically lethal with MMR defects.