The current view of cell-cycle control is that transitions between different phases of the cycle-G1, S, G2 and M are regulated by certain “check-points”. Two types of proteins are known to play critical roles. The cyclins and cyclin-dependent kinases or CDKs. One of the important check points lies in mid G1. In tumour cells, checkpoints in the cell cycle are often deregulated, because the genes encoding the cyclins or the CDKs may be mutated.