TKR, tyrosine kinase receptor; STR, seven transmembrane receptor; bFGF, basic fibroblast growth factor, an example of a mitogen; m1, PI-linked muscarinic receptor; m2, cAMP-inhibitory muscarinic receptor; ras, “small” mitogenic G protein; G16, Go, Gq, G12, PLC/PI-linked G proteins; Gi, cAMP-linked G protein. Other abbreviations are described in the text.