Protein phosphatase 1 inhibits p53 signaling by dephosphorylating and stabilizing Mdmx

► Protein phosphatase 1 (PP1) inhibits the p53 signaling in the DNA damage response. ► PP1 interacts with and dephosphorylates Mdmx at Ser367. ► Dephosphorylation of Mdmx by PP1 stabilizes Mdmx. ► PP1 inhibits Mdm2-Mdmx interaction in the DNA damage response.