Activated ATM phosphorylates CHEK2 (CHK2, Cds1) on threonine residue T68 (Matsuoka et al. 2000, Melchionna et al. 2000). The presence of BRCA1 and TP53BP1 positively regulates ATM-mediated phosphorylation of CHEK2 (Wang et al. 2002, Foray et al. 2003). ATM-mediated phosphorylation causes formation of CHEK2 dimers and dissociation of CHEK2 from chromatin (Li and Stern 2005).