CD274 and neoplasm: ,11 Differences in the immune tumour microenvironment have also been documented between primary and metastatic BCs, with MBC shown to have a lower TIL content and programmed death-ligand 1 (PD-L1) positivity compared to primary BCs as well as being more immunologically inert.12, 13, 14, 15, 16 Within these studies, BCBMs have been under-represented with only 3-21 cases included.12, 13, 14, 15, 16 A recent study of 93 paired primary BC and BCBM found fewer TILs in BCBMs with no differences observed with regard to PD-L1 expression by immunohistochemistry (IHC).17