562 Chapter 17 • Genetic Regulation of Cell Number: Normal and Cancer Cells has been uncoupled from its normal regulatory path-way, leading to its continuous unregulated expression. The continuously expressed protein product of an oncogene is called an oncoprotein. Several categories of oncogenes have been identiFed according to the differ-ent ways in which the regulatory functions have been uncoupled. PROTEIN DOMAIN DELETION OF A MITOGEN RE-CEPTOR TYROSINE KINASE A growth factor is not necessary to initiate the cell proliferation pathway if its receptor has been altered appropriately. The v-erbB oncogene is a mutated gene in a tumor-producing virus that infects birds. It encodes a mutated form of a recep-tor tyrosine kinase known as the EG±R, a receptor for the epidermal growth-factor ligand (±igure 17-17). Sev-MESSAGE The wild-type counterparts of oncogenes act to positively control the cell cycle or negatively control apoptosis. The following examples illustrate some of the ways
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