Proto-oncogenes and tumor suppressor genes, cancer biologists discovered, sit at the hubs of such signaling pathways. Ras, for instance, activates a protein called Mek. Mek in turn activates Erk, which, through several intermediary steps, ultimately accelerates cell division. This cascade of steps, called the Ras-Mek-Erk pathway—is tightly regulated in normal cells, thereby ensuring tightly regulated cell division. In cancer cells, activated “Ras” chronically and permanently activates Mek, which permanently activates Erk, resulting in uncontrolled cell division—pathological mitosis.
