The p53-Mdm2 Feedback Loop. (A) p53 transcriptionally activates Mdm2, while Mdm2 degrades p53. (B) The p53 and Mdm2 levels oscillate upon DNA damage . (C) Under normal conditions, Mdm2 is not suppressed by ATM and the intact feedback rejects disturbances. Upon DNA damage when p53 oscillates, the feedback is suppressed to allow p53 oscillation.