​Describe, using specific examples, the differences in how a growth factor gene and a tumor suppressor gene can become oncogenes.

Growth factor gene products stimulate cell division. Thus, a mutation that increases the activity or creates an overabundance of the gene product can transform the gene into an oncogene. For example, one proto-oncogene, EGFR, encodes a plasma membrane receptor for EGF (epidermal growth factor). EGF is a growth factor. When the EGF receptor binds to EGF, it becomes activated and triggers the cell to begin mitosis. When EGF is not present, the receptor is in an inactive form and does not trigger mitosis. Tumor-causing mutations in EGFR change the receptor so that it stimulates mitosis even in the absence of EGF.
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Tumor suppressor gene products prevent tumor formation. These genes become oncogenes when mutations ruin the function of their products. The protein products of checkpoint genes called BRCA1 and BRCA2 are tumor suppressors: Mutations in these genes give rise to neoplasms in the breast, prostate, ovary, and other tissues. The BRCA gene products help maintain the structure and number of chromosomes during mitosis, and they participate directly in DNA repair. BRCA gene products also bind to hormone receptors that are particularly abundant on cells of breast and ovarian tissues; the binding suppresses transcription of growth factor genes in these cells. When a mutation alters a BRCA gene so that its product cannot bind to these hormone receptors, the cells overproduce growth factors-an outcome associated with neoplasm formation.