Among several immunotherapies, the anti-PD1 monoclonal antibody pembrolizumab is the first to be approved as a subsequent line of therapy for unresectable advanced, recurrent, or metastatic disease characterized by MSI-H/dMMR, PD-L1 combined positive score (CPS) ≥ 1, or tumor mutational burden (TMB) ≥ 10 mutations/megabases [30]. The gene discussed is PDCD1; the disease is neoplasm.