PDCD1 and melanoma: Until recently, Food and Drug Administration (FDA)-approved therapeutic agents targeting the programmed cell death protein 1 (PD-1)/programmed cell death ligand 1 (PD-L1) axis have achieved impressive results in clinical immune checkpoint inhibitor (ICI) treatments, and serve as a crucial treatment for increasing cancer types, such as advanced melanoma (MM), non-small cell lung cancer (NSCLC), and colorectal carcinoma (CRC) (Constantinidou et al., 2019).