PDCD1 and neoplasm: Taking our PD-1 results into account, cats with rare mammary carcinomas (e.g., mucinous carcinomas, adenosquamous carcinomas and carcinosarcomas) could be strong candidates for immunotherapy with ICI, namely antibodies directed against the PD-1/PD-L1/PD-L2 axis, which could reactivate the functions of T lymphocytes, restoring the anti-tumor activity [48].