PDCD1 and melanoma: In addition to APCs, PD-1 ligands are expressed in various human cancer tissues [9], and PD-L1 expression in resected cancer tissues was correlated with postoperative survival time in ovarian cancer [10], malignant melanoma [11], esophageal cancer [12], renal cell carcinoma (RCC) [13], pancreatic cancer [14], and urothelial cancer [15], for example.