This prevents release of E2F Transcription Factor which is necessary for the cell to progress into the S-phase. Other signals are generated extracellularly, often by other cells. If they arise from nearby cells they are part of the paracrine system and are referred to as growth factors. These are generally proteins that cannot move across the cell membrane. The general term for these signaling molecules is ligands .
Protein ligands require a transmembrane receptor site to bind to.
Growth factors stimulate Mitosis. RTKs may cause a sequence of proteins to be activated leading to the activation of Transcription Factors. G-proteins have natural GTPase activity and soon degrade the GTP to GDP thereby inactivating themselves. Activated Ras propagates a signal that promotes cell proliferation. Cancer is a result of cell division occurring in an uncontrolled fashion thereby leading to tumor production.