Role of FLT3 in regulation of multipotent lymphoid-primed progenitors. BM cells from 2- and 19-week-old WT, Fl^−/−, and Il7r^−/− mice were stained with antibodies against lineage-specific antigens (Lin), KIT, CD34, SCA1, and FLT3. Lymphoid-primed multipotent progenitors (LMPPs) in WT mice were identified as the Lin⁻SCA1⁺ KIT⁺ cells with the highest 25% of FLT3 expression, as previously described.¹⁵  (A) Representative FACS profiles of each genotype of 19-week-old mice with gates for LMPPs set based on analysis in WT mice (numbers indicate mean percentages of total LSK cells, from 4-6 mice). (B) Frequencies of LMPPs per total BM cells (2 femora and 2 tibiae) in age-matched 2- and 19-week-old WT, Fl^−/−, and Il7r^−/− mice. Data represent mean (SD) values of 4 to 6 age-matched mice of each genotype. (C) Quantitative PCR data of indicated genes in FACS-sorted LSKFLT3⁺ cells from WT and Fl^−/− mice. LSKFLT3⁺ cells were sorted as LSKFLT3^hi LMPPs in the case of WT mice as indicated in panel A, whereas for Fl^−/− mice the 25% highest FLT3-expressing LSK cells were sorted. Lin⁻KIT⁺ BM cells (representing predominantly different lineage-committed progenitors) were used as controls. Data (normalized to the expression of HPRT) are mean (SD) values from 2 independent experiments, each performed in triplicates.