Anti-PD-1 antibodies, including pembrolizumab, have many side effects ranging from leukopenia, nausea, and fatigue, which are commonly seen with other anti-cancer drugs, to side effects attributed to the autoimmune response to this drug like hypothyroidism, type 1 diabetes, hypopituitarism, colitis, and drug-induced pneumonitis [4, 5]. The gene discussed is PDCD1; the disease is colitis.