TFRC and neoplasm: Apoferritin, an unloaded natural iron protein with a cage-like structure and transferrin receptor 1 (TfR1) targeting ability, was loaded with melanin nanoparticles (AMF), 64Cu2+ and Fe3+ to image HT29, high TfR1 expression, and HepG2, low TfR1 expression, in tumor-bearing mice.