The data obtained also revealed a positive correlation between serum PD-1 and PD-L1 levels in cats with mammary carcinoma (r = 0.780, p < 0.0001, Figure 2A), particularly in those exhibiting HER2-positive (r = 0.786, p = 0.03, Figure 2B) or TN normal-like tumor subtypes (r = 0.857, p = 0.03, Figure 2C). This evidence concerns the gene PDCD1 and neoplasm.