a number of mutations have been described in the g protein ras that have profound effects on its activity. for example, some mutations greatly increase the affinity of the g protein for gdp, making it very difficult for gdp to be exchanged for gtp. other mutations prevent the gtpase activity of ras, preventing it from causing the hydrolysis of gtp to gdp. which of the scenarios would result in a persisting proliferation response to growth factor receptor activation after the ligand is no longer binding to its receptor kinase?