The Authors evaluated the frequency of 17 SNPs in 4 genes (PD-1, PD-L1, IDO1 and VEGFR2), selected for their functional and clinical relevance, in a cohort of 98 patients, affected by different types of cancer and treated with anti-PD-1 or anti-PD-L1 monotherapy alone. The gene discussed is PDCD1; the disease is cancer.