Inhibition of cell proliferation and KIT phosphorylation by PD180970. (A) KIT juxtamembrane and activation loop mutant-expressing cell lines were assessed for cell proliferation by MTT assay at 72 hours in the presence of escalating doses of PD180970 and graphed as the average of 3 independent experiments plated in triplicate. (B) KIT immunoprecipitates from the same cell lines were blotted for phosphotyrosine content in the presence of escalating doses of PD180970. Total KIT immunoblots were performed as a loading control. IC₅₀ values were calculated as the average of 3 independent experiments. Representative gels are shown.