A phase 2 study evaluating the clinical activity of the PD-1 inhibitor pembrolizumab in patients with colorectal cancer demonstrated an immune-related PFS rates of 78% in patients with MMR-deficient cancer and 11% in patients with MMR-proficient cancer, demonstrating that MMR status predicts the clinical benefit of pembrolizumab [138]. The gene discussed is MRC1; the disease is colorectal cancer.