KITD814V-induced ligand-independent growth and promiscuous cooperation with other cytokines is inhibited in the absence of p85α or Rac2 GTPase in MCps. KITD814V-expressing wild-type, p85α^−/−, Rac2^−/−, or Rac2^−/−:p85α^−/− MCps were subjected to a thymidine incorporation assay in the absence of cytokine stimulation (NO GF) or in the presence of IL-4 10 ng/mL, IL-10 10 ng/mL, or SCF 50 ng/mL plus IL-4 10 ng/mL. After 48 hours of culture, cells were pulsed with [³H] thymidine for an additional 6 hours. Bars denote the mean thymidine incorporation (cpm ± SD) from 1 of 2 independent experiments performed in quadruplicate. *P < .05, KITD814V-expressing p85α^−/−, Rac2^−/−, or p85α^−/−:Rac2^−/− cells versus wild-type cells.