PDCD1 and melanoma: Perhaps the earliest and most widely recognized predictive biomarker of response to PD-1/PD-L1 blockade is PD-L1 expression, for which there are 4 FDA-approved assays of PD-L1 expression by IHC (Table 1) to help guide treatment decisions for nivolumab in advanced NSCLC or melanoma (Dako 28-8), pembrolizumab in advanced NSCLC (Dako 22C3), atezolizumab in advanced urothelial carcinoma or NSCLC (Ventana SP142), and durvalumab in advanced urothelial carcinoma (Ventana SP263) [20, 24, 27, 28, 35, 37, 38].