KL, as well as FL, are required for optimal proB-cell development from flt3⁻ candidate murine stem cells. Lin^−/loSca1⁺c-kit⁺ cells were sorted into flt3⁺ and flt3⁻ populations and cultured in the presence of the indicated cytokines at a density of 1 cell per well. Fifteen to 25 days after initiation of culture, individual colonies were picked and analyzed by flow cytometry to verify the presence of proB-cells, as defined by combined B220 and CD19 expression. No proB-colonies were found in response to KL + IL-7. Results represent the mean (+SEM) of 4 individual experiments. Cells stimulated with an optimal combination of cytokines (KL + FL + MGDF + IL-3 + G-CSF + GM-CSF + Epo) served as a control for total number of in vitro clonogenic progenitors. Three independent single-cell experiments with flt3⁺ and flt3⁻ cells showed a high cloning frequency, 98 (1)% and 97 (1)%, respectively. Thus, the data are presented as percent B-lymphoid colonies of the total number of clonogenic progenitors.