APOE and neoplasm: For the former, researchers generally adopt a step-by-step targeting method, that is, to modify the transporters that can specifically target endothelial cells (such as transferrin, lactoferrin, apolipoprotein E, penetrating peptide) and the components that specifically target tumor cells (such as folic acid, hyaluronic acid, ursolic acid) to break through the BBB and target tumor tissue8.