One important protein in the cell cycle is p53, a transcription factor
(see the Genes and Development unit) that binds to DNA, activating
transcription of a protein called p21. P21 blocks the activity of a cyclin-
dependent kinase required for progression through G1. This block
allows time for the cell to repair the DNA before it is replicated. If the
DNA damage is so extensive that it cannot be repaired, p53 triggers
the cell to commit suicide. The most common mutation leading to
cancer is in the gene that makes p53. Li-Fraumeni syndrome, an
inherited predisposition to multiple cancers, results from a germ line
(egg or sperm) mutation in p53. Other proteins that stop the cell cycle
by inhibiting cyclin dependent kinases are p16 and RB. All of these
proteins, including p53, are tumor suppressors.
Cancer cells do not stop dividing, so what stops a normal cell from
dividing? In terms of cell division, normal cells differ from cancer cells
in at least four ways.