Although most studies on breast cancer BM have been performed on the triple negative breast cancer cell lines [155], it has been demonstrated that BM of HER2-positive breast cancer preserves the integrity of BBB through expressing transporters (GLUT1 and BCRP) in the intratumor microvessels compared to the triple negative breast cancer that shows a negative correlation with these transporter expressions. The gene discussed is ABCG2; the disease is breast cancer.