The main check points are present at G1/S, G2/M and M. Each check point is controlled by the mutual interaction between cyclin and cyclin dependent protein kinase. p53 gene products are known to control many events through G1/S and G2/M checkpoints.

 

So what if any of these events goes wrong?

Disregulation of cell cycle and control mechanism give rise to tumor. After certain number of cell division, every cell enters into the G0 phase and ceases the cell division. In the case of tumor, cells lost the control mechanism and multiply infinitely to give rise to cell mass. These cells are taking nutrition but not performing functions. Retinoblastoma (RB), p⁵³ are the crucial cellular factor responsible for cell cycle control and play crucial role in tumor development.