Another critical protein in regulating the cell cycle is the tumor suppressor protein p53 . In fact, p53 is the most frequently disrupted gene in cancer, illustrating its importance.
Apoptosis is basically cellular suicide.
Now, p53 protein levels are normally kept very low within the cell. Mdm2 then inactivates p53, turning it off. As you can imagine, if you want to increase p53 protein levels within the cell, you are going to have to inhibit Mdm2. Well, it depends on the stimulation. So, when p53 is phosphorylated here, it can no longer bind to Mdm2.