The ras protein is a gtpase that functions in many growth factor–signaling pathways. in its active form, with gtp bound, it transmits a downstream signal that leads to cell proliferation; in its inactive form, with gdp bound, the signal is not transmitted. mutations in the gene for ras are found in many cancers. of the choices below, which alteration of ras activity is most likely to contribute to the uncontrolled growth of cancer cells?