G Proteins provide signal transduction mechanisms to seven transmembrane receptors. Recent studies have indicated that the a-subunits as well as the bg-subunits of these proteins regulate several critical signaling pathways involved in cell proliferation, di erentiation and apoptosis. Of the 17 a-subunits that have been cloned, at least ten of them have been shown to couple mitogenic signaling in ®broblast cells. Activating mutations in Ga s , Ga i2 , and Ga 12 have been correlated with di erent types of tumors. In addition, the ability of the bg-subunits to activate mitogenic pathways in di erent cell-types has been de®ned. The present review brie¯y summarizes the diverse and novel signaling pathways regulated by the a-as well as the bg-subunits of G proteins in regulating cell proliferation.