PDCD1 and neoplasm: CTLA-4 expression on tumor cells has been shown to be associated with response to the anti-CTLA-4 inhibitor, ipilimumab, in melanoma.12 Several studies have also highlighted the significant correlation between PD-L1 expression and clinical outcomes on anti-PD-1-based immunotherapies.13–15 However, the clinical utility of PD-L1 as a biomarker of response to anti-PD-1-based therapies has been limited by its heterogenous expression in melanoma,16 and the complex immunobiology underlying response and resistance.