563 17.5 Cancer: the genetics of aberrant cell number regulation eral parts of the normal EGFR are missing in the mu-tated form. The mutated form of the EGFR lacks the extracellular, ligand-binding domain as well as some reg-ulatory components of the cytoplasmic domain. As a consequence, the truncated EGFR oncoprotein is able to dimerize even in the absence of the EGF ligand. The EGFR oncoprotein dimer is always autophosphorylated through its tyrosine kinase activity and so continuously initiates a signal-transduction cascade that promotes cell proliferation. A PROTEIN FUSION INVOLVING AN INTRACELLULAR SIGNAL TRANSDUCER Perhaps the most remarkable type of structurally altered oncoprotein is caused by a gene fusion. The classic example emerged from the re-sults of studies of the Philadelphia chromosome, which, as already mentioned, is a translocation between chromo-somes 9 and 22 that is a diagnostic feature of chronic myelogenous leukemia. Recombinant DNA methods have shown that the breakpoints of the Philadelphia
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