Cellular replication is an essential process by which a single-celled organism develops into a multicelluar being. Replication also allows the body to renew itself. The cellular cycle of eukaryotic cells consists of four phases: G1 phase, S phase, G2 phase and M phase. Cells increase in size during G1 phase, which is followed by DNA replication in S phase. Protein synthesis and the production of microtubules occurs during G2 phase, which is then followed by mitosis. Mitosis is the briefest phase of the cell cycle, however is extremely important. During this period, nuclear and cytoplasmic division occurs, concluding with cyotkenesis. When cells are not actively dividing they are considered to be in a state of quiescence, known as the G0 phase. The passage of a cell through the cell cycle is controlled by various cytoplasmic proteins in order to assure quality control.