CD274 and transitional cell carcinoma: Several types of canine malignant tumours also express PD-L1, including squamous cell carcinoma, nasal adenocarcinoma, transitional cell carcinoma, osteosarcoma, and malignant melanoma, suggesting that canine tumours have an immune evasion mechanism through PD-L1 overexpression [28,29,30,31,32,33,34,35,36].