PDCD1 and neoplasm: Tumor mutational burden, a potential surrogate for tumor neoantigens that can be recognized by the immune system, is one such biomarker, leading to the first ever histology-agnostic FDA approval of the PD-1 inhibitor pembrolizumab for mismatch repair deficient tumors of any histology (14, 15), though there is increasing recognition that the types and functional nature of mutations may be as important as the number of mutations present (16).