Serum levels of soluble PD-1 and PD-L1 were significantly higher in cats with human epidermal growth factor receptor 2 positive (HER2+) and triple-negative (negative for estrogen and progesterone receptors and HER2) normal-like mammary carcinomas than in cats with other molecular subtypes of mammary carcinoma and those without mammary neoplasm [71]. Here, PGR is linked to breast carcinoma.