The addition of animal serum or specific protein growth factors to quiescent, serum-deprived vertebrate cells in culture activates a wide variety of biochemical responses within minutes, which are followed in 5–10h by the initiation of DNA synthesis and then by mitosis. The quintessential early and continuing activation step for the increase in DNA synthesis is the increased initiation rate of protein synthesis, which must be continuously maintained throughout the G1 phase for advancement into S. The aggregate of biochemical reactions to growth factors is called the coordinate response, to indicate that many related and unrelated processes are orchestrated to repetitively reproduce cells. It is, however, crucial to recognize that the coordinate response can be induced for one or more rounds of replication by a variety of non-specific and quasi-specific membrane effectors. The logic of considering this framework of events in growth control implied that a single multi-target second messenger plays a central role in coordinating the events of the overall response. The same reasoning suggested that free Mg²⁺ is the unifying regulatory element in that response which includes protein kinase pathways, and that the cytoplasmic activity of Mg²⁺ increases with the binding of growth factors to their receptors in the cell membrane, or of less specific perturbations of the membrane. Experimental support of this conclusion is presented here and is represented in the MMM model of cell proliferation control.