The prior belief was that growth control is exerted at the end of a cycle. In contrast, two major researches on this question showed it to be exerted in late G1 phase This process, at about two hours prior to initiation of DNA synthesis in mammalian cells at, the Restriction (R) Point discovered by Pardee in 1974.65 Lee Hartwell at this time demonstrated that growth of cycle-regulating yeast mutants that he isolated is similarly regulated at “START” in G1.66 R and START are the first demonstrations of what were later named “checkpoints”. Thus, a wholly different set of events came into consideration, and it involved molecules soon identified. Numerous intracellular mechanisms regulate eukaryotic cell cycling. Protein synthesis is one such essential process.67 Rapid protein synthesis is needed to enter S-phase, which suggests the requirement for a growth regulating protein with a short half-life. A sensor for protein synthesis and cell growth is the kinase mTor; locking its activity with rapamycin stops cells in G1.68 This led to discovery of control of the R protein by its synthetic vs. degradation rates. Only one protein (p68) of the many detected on 2-D gels had three characteristics of R. It increases in G1, has a short half-life, and is elevated in cancer cells.69 R protein might be cyclin E, one of the cyclin family of regulatory proteins that are central to cell cycle control. Several proteins including structural actin also increase after cells enter their proliferation cycle.