Immune checkpoint inhibitors have received broad and significant interest because of their ability to generate durable responses in intractable malignant tumors and for improvements in overall survival [11,12]. The number of clinical trials using PD-1 and PD-L1 inhibitors has increased by nearly 600% between just 2015 and 2017 [13] and the market is expected to grow similarly [10]. Currently, the most used ICIs are CTLA-4 inhibitors (ipilimumab), PD-1 inhibitors (pembrolizumab and nivolumab), and PD-L1 inhibitors (atezolizumab, avelumab, and durvalumab). The gene discussed is PDCD1; the disease is cancer.