FL-dependent signaling and progenitor expansion are enhanced in Lnk^−/− mice. (A-B) Primary BM cells from Lnk^+/+ and Lnk^−/− mice were cytokine starved for 6 hours and stimulated with FL for the indicated times. After fixation and permeabilization, cells were sorted by FLT3 expression. Phosphorylation of ERK was measured in FLT3⁺ cells using flow cytometry, and the mean intensity is shown. (C) Methylcellulose colony forming assays were performed with either primary BM cells or Lin⁻ BM cells from Lnk^+/+ and Lnk^−/− mice in the presence of either FL + IL6 or SCF. Assays were performed in duplicate and data represent the means ± SD of 4 independent mice. (D) Limiting-dilution assay of sorted LMPPs (Flt3⁺Sca1⁺Kit⁺Lin⁻) was performed in the presence of FL and IL6. The frequencies of FL-responsive LMPPs (with 95% confidence limits) were 1 in 9 (7-11) and 1 in 18 (15-22) within the sorted cells isolated from Lnk^−/− and Lnk^+/+ mice, respectively.