When compared with primary breast cancer, breast cancer BrM tends to have lower infiltration of immune cells (macrophages, microglia, lymphocytes, and monocytes), lower protein and gene expression of immune activation markers (CD27, T cell immunoglobulin and mucin-domain containing-3 (Tim-3), and CD137), and lower expression of immune-related genes (PD-L1 and CTLA-4) (Schlam et al., 2021; Giannoudis et al., 2022). Here, CD27 is linked to breast cancer.