p53 acts as a tumor suppressor in many tumor types, induces growth arrest or apoptosis depending on the physiological circumstances and cell type. Involved in cell cycle regulation as a trans-activator that acts to negatively regulate cell division by controlling a set of genes required for this process. Prevents CDK7 kinase activity when associated to CAK complex in response to DNA damage, thus stopping cell cycle progression. E3 ligase MDM2 mediated ubiquitin conjugation of p53 and subsequent p53 down-regulation via proteasomal degradation interrupts this crucial role, contributing to tumorogenesis (and cancer).