PDCD1 and osteosarcoma: Markers of potential response to immunotherapy such as programmed cell death 1 (PD-1) and its ligand, programmed cell death ligand 1 (PD-L1), have also been found to be overexpressed in cancer cells and cancer infiltrating lymphocytes in oral melanoma, lymphoma, osteosarcoma, and urothelial carcinoma in dogs [31,32,33,34,35].