TF and brain neoplasm: IHC staining of U87MG and GL261 brain tumors demonstrated transferrin receptor expression in tumor-associated blood vessels and tumor cells (Fig. 2a) and intravital imaging showing uptake of Tf-NPs into the endothelium of brain microvessels (Fig. 1e) and at the tumor site (Fig. 2e), suggests that functionalization of our NPs with transferrin allows for receptor-mediated transcytosis despite having an average diameter of 140 nm.