Scientists have noticed that during a certain phase of cell division, the two halves of a normal, dividing cell look and act like two cells migrating (or moving) away from each other. The concentration of a chemical called PIP3 is low at the cleavage furrow, or the 'backs' of what will be two new cells. PIP3 concentration is high on the sides that represent the 'fronts' of what will be two new cells. Substances called CDK's control the breakdown of the nuclear membrane during mitosis. If the genes coding for these substances are mutated, mitosis can run out of control and cells will not stop dividing. Which specific phase in the cell cycle is regulated by CDK's?