CHEK2 (Chk2) phosphorylates TP53 (p53) at serine residue S20 (Hirao et al. 2000, Shieh et al. 2000, Chehab et al. 2000). Phosphorylation of TP53 at serine residue S20 is necessary for DNA damage-induced TP53 stabilization as it compromises the interaction of TP53 with the ubiquitin ligase MDM2 (Chehab et al. 1999, Chehab et al. 2000). S20 phosphorylation is also required for the induction of TP53-dependent transcripts in response to DNA damage (Hirao et al. 2000).