PDCD1 and cancer: Inhibitors targeting the T cell immune checkpoint molecules programmed cell death protein 1 (PD-1) and cytotoxic T-lymphocyte associated protein 4 (CTLA-4) have been monumentally impactful for the treatment of various cancers, including metastatic melanoma [3], non-small cell lung cancer [4], and head and neck squamous cell carcinoma [5].