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. This evidence concerns the gene PDCD1 and breast cancer.