PD-1 is largely expressed on both CD8+ (70.9–96.6%) and CD4+ (80.2–96.8%) tumour-infiltrated lymphocytes (TILs) obtained from canine oral melanoma, demonstrating upregulation of PD-1 possibly leading to functional exhaustion of TILs in this cancer type [55]. The gene discussed is CD8A; the disease is neoplasm.