FIG. 7.

Model depicting a potential regulatory loop involving p53 and CHK1. Genotoxic stress activates CHK1 (and other factors), leading to the accumulation of p53 protein. Stabilized p53 transcriptionaly activates the Hdm-2 and p21 genes. Hdm-2 negatively affects p53 by targeting it to the ubiquitination machinery, while p21 through pRB transcriptionally represses CHK1, thus both controlling p53 levels and partially releasing the G₂ checkpoint. CHK1∗ represents the activated form of the kinase.