Cells coordinate their division by using chemical signals with one another using unique proteins known as cyclins. These signals function as switches, indicating when cells should begin division and subsequently when they should stop dividing. Cell cycle progression is regulated by cyclin-dependent kinases (Cdks) and their subunits. At each phase of the cell cycle, cyclin-Cdk complexes ensure that division continues only under the proper conditions and with sufficient growth in the cell. For example, M-phase cyclins make up the M-CDK complexes, and initiate the cell’s entry into mitosis. Certain cells, such as skin cells, are continuously dividing. This is because humans need to form new skin cells to take the place of the old skin cells that are lost. Cells may also receive signals from growth factors and hormones, which induce cell division. Growth factors have a significant role in promoting the growth of specific tissues in the body. Each growth factor attaches to a specific receptor on the cell to initiate signaling pathways. Receptor-tyrosine-kinases (RTKs) play the most vital roles in cell cycle regulation as growth factors. Growth factors also play a role in cell differentiation, and abnormal activation of growth factor pathways can lead to excessive cell proliferation.