“…Several enzymes, such as ATPases, 5'-nucleotidases, proteases, and protein kinases, are located in or on the plasma membrane (Chiang et al, 1982;Kang et al, 1978Kang et al, , 1979Kinzel and Mueller, 1973;Kubler et al, 1982;MatManus and Whitfield, 1981;McPherson and Ramachandran, 1980;Mastro, 1979;Mastro and Rozengurt, 1976;Schlaeger and Kohler, 1976;Sommarin et al, 1981;Stanley et al, 1982;Wen et al, 1985). Of all these enzymes, the protein kinases seem to be especially important for proliferation, because several growth factors, such as EGF (Ushiro and Cohen, 1980), IGF-1 (Jacobs et al, 1983;Rubin et al, 1983), insulin (Kasuga et al, 1982), and PDGF (Ek et al, 1982;Nishimura et al, 19821, as well as the Avian sarcoma virus's mitogenid oncogenic pp6OwSrc product (Collett and Erikson, 1978;Whitfield, 1984, 1985), stimulate the phosphorylation of plasma membrane proteins, and a burst of membrane protein phosphorylation precedes the onset of DNA replication in Swiss albino 3T3 mouse cells and regenerating rat liver cells Mastro and Rozengurt, 1976).…”