MDM2 and cancer: Cellular stress signals result in p53 stabilization by post-translational modifications, such as phosphorylation of Ser-20 24 and/or inhibition of the MDM2-p53 interaction.25 Overexpression of mutant and/or wild-type (WT) p53 is often detected in human cancers and has been used as an important biomarker for cancer diagnostics/therapeutics.26–30 Several genetically encoded biosensors have been developed for imaging p53.31,32 However, difficulties in controlling their in situ expression levels, and the need for in situ genetic manipulation limit many of their applications.