PDCD1LG2 and head and neck squamous cell carcinoma: In human oncology, many studies have identified PD-L2 expression in various types of tumors, including, but not limited to, breast carcinoma [17], hepatocellular carcinoma [63], lung carcinoma [64], gastric carcinoma [65] and head and neck squamous cell carcinoma (HNSCC) [66].