The process of cell proliferation is tightly controlled.
Cancer develops when cells start to proliferate and escape that control.
Cancer is the result of a rupture of the exquisite equilibrium established between the proliferation of the cells and their death. A cell should exclusively divide when the conditions require that it does, and this is signaled through a battery of messengers called growth factors. Even in the presence of those factors, a very complex control mechanism checks whether the moment for division is appropriate, and every step of replication of the cell is followed by checkpoints that determine if it has been successfully completed. For this reason, virtually all of the proteins implicated in these processes can, when altered, be the cause of cancer. They are then called protooncogenes in their normal form, and oncogenes in their altered manifestation.