CD274 and breast cancer: Considering the above evidence and the fact that rare feline mammary tumors have not been investigated either due to their relative rarity, or probably to limited inter-observer agreement in their diagnosis, in this study we aimed to (i) evaluate the expression of PD-1, PD-L1 and PD-L2 in the tumor cells and TILs (intratumoral and stromal) of rare feline mammary carcinomas and (ii) explore possible associations between IHC scores and various clinicopathological characteristics.