Abstract
In this chapter, we will present the transcription factor p53, which is encoded by the most prominent cancer driver gene, the tumor suppressor gene TP53 . Activated p53 proteins regulate hundreds of target genes that control cell fate controlling processes, such as DNA damage repair, cell cycle arrest, apoptosis and senescence. The aberrant activity of proteins ruling the cell cycle, such as the tumor suppressor protein RB1 and several CDKNs as well as oncogene-encoded cyclins and CDKs, leads to loss of proliferative control. The deletion of one or both alleles of a tumor suppressor gene (two hit model) or their gradual inactivation by transcriptional or epigenetic processes (continuum model) can explain both the inheritance as well as the mechanisms of cancer onset.