MDM2 and neoplasm: To the best of our knowledge, there have been no published reports of radiotracer development for PET imaging of MDM2 at the protein level; however, Fu et al. demonstrated that a 99mTc-labeled antisense oligonucleotide targeting MDM2 mRNA accumulated in MCF-7 tumor xenografts by single-photon emission computed tomography (SPECT) imaging, with a tumor-to-muscle ratio of ~4 at 10 h post injection [20,21].