Ras is a GTP-binding protein that is often defective in cancer cells. A signal from a growth factor through a receptor tyrosine kinase often stimulates normal cells to divide. When the receptor tyrosine kinase binds the growth factor, Ras is stimulated to bind GTP. Ras in turn activates proteins that promote cell proliferation. A common mutation in cancerous cells causes Ras to behave as though it were bound to GTP all the time.