Oncogenes, Tumor-Suppressor Genes, and Tumors For each of the following situations, provide a plausible explanation for how it could lead to unrestricted cell division. (a) Colon cancer cells often contain mutations in the gene encoding the prostaglandin E, receptor. PGE is a growth factor required for the division of cells in the gastrointestinal tract. (b) Kaposi sarcoma, a common tumor in people with untreated AIDS, is caused by a virus carrying a gene for a protein similar to the chemokine receptors CXCR1 and CXCR2. Chemokines are cell- specific growth factors. (e) Adenovirus, a tumor virus, carries a gene for the protein E1A, which binds to the retinoblastoma protein, pRb. (Hint: See Fig. 12-48.) (d) An important feature of many oncogenes and tumor suppressor genes is their cell-type speci- ficity. For example, mutations in the PGE, receptor are not typically found in lung tumors. Explain this observation. (Note that PGE, acts through a GPCR in the plasma membrane.)