Later studies pin-pointed that a single serine (Ser-15) was phosphorylated by ATM and phosphorylation of Ser-15 was rapidly-induced in IR-treated cells and this response was ATM-dependent (Canman et al, 1998; Banin et al, 1998 and Khanna et al, 1998). ATM also regulates the phosphorylation of p53 at other sites, especially Ser-20, by activating other serine/threonine kinases in response to IR (Chehab et al, 2000; Shieh et al, 2000; Hirao et al 2000). Phosphorylation of p53 at Ser-20 interferes with p53-MDM2 interaction. MDM2 is transcriptionally activated by p53 and is a negative regulator of p53 that targets it for degradation (Haupt et al, 1997; Kubbutat et al, 1997). In addition modification of MDM2 by ATM also affects p53 stabilization (Maya et al, 2001).