KITD814V-induced ligand-independent growth and promiscuous cooperation with other cytokines is inhibited in the absence of Rac2 GTPase in HSC/Ps. (A) Wild-type or Rac2^−/− bone marrow LDMNCs were transduced with a retrovirus expressing the empty vector or KITD814V. Transduction efficiency of various constructs in wild-type and Rac2^−/− progenitors is shown. (B) EGFP-positive cells in panel A were sorted to homogeneity and subjected to thymidine incorporation assay in the presence of indicated cytokines: no growth factor (NO GF), SCF 50 ng/mL, M-CSF 50 ng/mL, or G-CSF 20 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 3 independent experiments performed in quadruplicate. *P < .05, KITD814V-expressing Rac2^−/− cells versus wild-type cells in response to indicated stimuli.