CD274 and breast carcinoma: Given the limited data on PD-1 expression and the anecdotal data on PD-L1 expression in primary breast cancers compared to their matched distant metastases to date, we compared immunohistochemical expression levels of PD-1 and PD-L1 in primary tumors and their matched distant metastases in a large group of metastatic breast cancer patients, and evaluated prognostic values.