In this regard, Székely et al. [27] have reported that the number of tumor-infiltrating lymphocytes (TILs) and the protein expression of programmed death ligand 1 (PD-L1) are substantially lower in samples obtained from metastases derived from patients diagnosed with breast cancer than in samples obtained from patients diagnosed with primary tumors, as well as a significantly lower expression of CD27 and its ligands CD70, CD29, and CD40L, which are necessary for T cell activation. Here, CD27 is linked to breast carcinoma.