The distribution of systemically administered chemotherapies is hampered by the BTB, which is frequently disrupted in patients with brain metastases.45,46 We analyzed over 2000 brain metastases in different preclinical models of metastatic breast cancer (human 231-BR-HER2 and murine 4T1-BR5) and found in over 89% of lesions, there was a partial compromise in BTB permeability. Here, ERBB2 is linked to breast cancer.