TP53 and neoplasm: MDM2 keeps p53′s activity low under normal conditions, and since p53 induces the expression of MDM2, both proteins are kept at low concentrations.1, 2 Mouse studies underlined the importance of this feedback loop mechanism: MDM2 knockout is embryonic lethal due to uncontrolled p53 activity levels and can be rescued by simultaneous p53 knockout.3, 4, 5 Upon cellular stress such as DNA damage, p53 is uncoupled from MDM2, allowing it to carry out its anti-tumour functions by inducing the gene expression of proteins involved in DNA repair and apoptosis, depending on the type and degree of stress.