Only 5% administered dose of Trastuzumab, which targets HER2+ breast cancer, is found in brain lesions in preclinical mouse models.34 Clinically, the ratio of trastuzumab in serum to cerebrospinal fluid is 420:1.53 Often a factor limiting drug delivery to brain metastases is the high degree of expression of ABC efflux transporters at the BBB and BTB. This evidence concerns the gene ERBB2 and breast cancer.