Despite our incomplete comprehension of how growth factor-stimulation of cells is linked to the cell cycle and of how the G(1)/S checkpoint is linked to initiation in DNA replication there is an unparalleled wealth of experimental evidence to connect protein phosphorylation to molecular mechanisms of carcinogenesis. Many growth factors, growth factor receptors with tyrosine kinase activity (insulin receptor, EGFR, PDGFR, CSF1R, NGFR, HGFR), nonreceptor serine/threonine or tyrosine kinases (c-Raf-1, cMos, c-Abl, c-Src) and cyclin D1 are encoded by oncogenes mutated or overexpressed in a variety of human tumors; the physiological functions of oncoproteins that are involved in gene expression and replication (c-Jun, T-antigen, c-Myc, c-Myb) as well as p53, RB and CDK4 tumor suppressor proteins and replication factor A are also regulated by phosphorylation, ms genes transduce growth factor receptor signals to protein kinase C (PKC) or to c-Raf-1 triggering two different cascades of protein kinases, the PKC and MAPK signaling pathways both targeting nuclear proteins. Thus cancer can be considered as a disease of the signaling pathways.

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