FIGURE 24-20 Effects of oncogenic mutations in proto-oncogenes that encode cell-surface receptors. Left: A mutation that alters a single amino acid (valine to glutamine) in the transmembrane region of the HER2 receptor causes dimerization of the receptor, even in the absence of the normal EGF-related ligand, transforming it into the oncoprotein NEU, a constitutively active kinase. Right: A deletion that causes loss of the extracellular ligand-binding domain in the EGF receptor leads, for unknown reasons, to constitutive activation of the kinase activity of the resulting oncoprotein, ErbB.