In another trial, measurement of the serum-to-CSF ratio of trastuzumab in patients with HER2-positive breast cancer (start 420:1) showed an increase in trastuzumab after RTx (76:1) or LMD (49:1), indicating a disrupted blood–CSF barrier and therefore a potential for agents to enter this space [94]. This evidence concerns the gene ERBB2 and breast cancer.