Influence of cytokines on colony formation by CD34⁺⁺⁺kit⁺⁺, kit⁺, and kit^Lo/− cells transduced with c-kit cDNA. Sorted cells were prestimulated with full dosages of IL-3 (200 U), GM-CSF (200 U), Epo (1 U), and IL-6 (10 ng/mL) and then transduced withc-kit cDNA (M5g hkit Neo) or mock (M5g Neo) viruses as described in Materials and Methods. After washing, cells were assayed at 800 cells/mL for colony formation in semisolid culture in the presence of IL-3, GM-CSF, and Epo with SLF at 0 (I), 2 (II), 10 (III), and 50 (IV) ng per mL in the serum-depleted cultures as described in Materials and Methods. Results are expressed as mean ± SEM from 1 representative of 3 separate experiments. Significant differences from mock virus control are: ^aP < .01;^bP < .05.