TF and neoplasm: Studies using various tumor targeting ligands, e.g., folic acid 13, amino terminal fragment (ATF) peptide 14, RGD 15, transferrin (Tf) 16, and anti-HER2 antibodies 17, in different animal tumor models showed that active targeting did promote rapid and early binding of NPs to tumor vessels, although the reports on long-term tumor accumulation of NPs were inconsistent.