For example, (i) genetic mutations in COX2, EGFR, HBEG, ST6GALNAC5 (9), and MMP2, 3, 9 are found to be associated with the increased ability to penetrate the BBB (6, 10), particularly for breast cancer, the most studied primary cancer type in relevance to brain metastasis; and (ii) enhanced expressions of IL1b, HES5 (11), PCDH7 (12), γ-aminobutyric acid-related genes (13), and the activated Notch signaling pathway (7) can increase the ability of the arriving cancer cells to survive in the brain microenvironment. Here, PCDH7 is linked to cancer.