The survival, , and size of each cell in an animal are controlled by extracellular signal molecules secreted by neighboring and distant cells. Many of these signal molecules bind to a cell-surface and trigger various intracellular signaling pathways. one class of signal molecules, called , stimulates cell division by releasing the molecular brakes that keep cells in the or phase of the cell cycle. Members of a second class of signal molecules are called , because they stimulate cell growth and an increase in cell mass. The third class of signal molecules, called , inhibits by regulating members of the family of proteins. In addition to such stimulatory factors, some signal proteins such as act negatively on other cells, inhibiting their survival, growth, or proliferation.