PDCD1 and cancer: Immune checkpoint inhibition (ICI) improves survival in a growing number of cancers.1, 2, 3 ICI activates T cells by blocking two inhibitory T cell receptors: programmed cell death protein 1 (PD-1) with nivolumab (nivo) or pembrolizumab (pembro) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) with ipilimumab (ipi).