FL, but not KL, efficiently promotes development of B220⁺IL-7R⁺ cells from Lin^−/loSca1⁺c-kit⁺flt3⁺progenitors. A total of 2 to 10,000 Lin^−/loSca1⁺c-kit⁺flt3⁺(flt3⁺) and Lin^−/loSca1⁺c-kit⁺flt3⁻(flt3⁻) cells was cultured in serum-depleted medium supplemented with either KL or FL, as depicted in the figure. After 7 to 8 days in culture, cells were counted and evaluated for B220 and IL-7R expression. The figure shows 1 representative experiment of 3 individual experiments, whereas the mean (±SEM) is shown for each quadrant. Flt3⁻ cells cultured in FL did not survive and thus could not be investigated.