Figure 2: Regulation of the NG2 cell cycle. The main difference between the NG2 cell cycle (lower part) and classical cell cycle (upper part) lies in the fact that CDK2 is not essential in the earlier stage of the NG2 cell cycle because of the compensatory function of CDK4 (red dotted line box). Aside from the said observation, there are no further differences reported between the NG2 cell cycle and the classical cell cycle. Both the NG2 cell cycle and the classical cell cycle are regulated by cyclin-dependent kinase activating kinases (CAKs) and CDK-inhibitors (CKIs). The former mainly promote the progression of the NG2 cell cycle, whereas CKIs inhibit cell division or induce cell differentiation. The extracellular signals mainly act on CKIs or CAKs, thereby altering the fate of NG2 cells.