Loss of early thymic progenitors in adult mice double deficient in Fl and Il7r expression. (A) Thymocytes from 8- to 10-week-old mice were stained with antibodies against CD3, CD4, CD8, Mac-1, Gr-1, Ter119 and B220 (Lin), CD44, CD25, and KIT to establish relative distribution of DN1-DN4 stage thymocytes. Representative FACS profiles of CD44 and CD25 expression within Lin⁻ gate of each mouse genotype. Numbers in quadrants show mean percentages of total thymocytes in 6 to 7 mice. (B) Representative FACS profiles of KIT and CD25 expression within DN1 and DN2 thymic progenitors (gated within Lin⁻CD44⁺ cells) of each mouse genotype. Numbers show mean percentage of DN1 and DN2 cells expressing KIT. (C) Total number of all DN thymic subsets per thymus. Data represent mean (SD) values of 6 to 7 age-matched mice of each genotype. 0 indicates undetectable levels. *P < .01 comparing Il7r^−/− and Fl^−/−Il7r^−/− mice; **P < .01 comparing Fl^−/− and WT mice. (D) Representative FACS profiles of CD4 and CD8 expression in the thymus of each mouse genotype (numbers in quadrants show mean percentages of total thymocytes in 6–8 mice). (E) Total number of DP and SP thymocytes per thymus. Data represent mean (SD) values of 6 to 8 age-matched mice of each genotype. *P < .01 comparing Il7r^−/− and Fl^−/−Il7r^−/− mice. (F) FL expression in FACS-sorted thymic stromal cells, DN, DP, and SP thymocytes as measured with quantitative PCR. Data (normalized to the expression of β-actin) from one experiment in which thymuses from 4 WT mice were pooled.