Early thymic progenitors are reduced but present in Fl^−/−Il7r^−/− mice during fetal and early postnatal stages of development. Total thymic cellularity was determined in 17- to 18-day-old fetuses and in 2-week-old WT, Il7r^−/−, Fl^−/−, and Fl^−/−Il7r^−/− mice. (A) Total number of DN1-DN4 populations per thymus of 17- to 18-day fetuses. (B) Total number of DP and SP cells per thymus of 17- to 18-day fetuses. (C) Total number of DN1-DN4 populations per thymus of 2-week-old mice. (D) Total number of DP and SP cells per thymus of 2-week-old mice. (E) Total, CD4, and CD8 T cells in peripheral blood of 2-week-old mice. All data represent mean (SD) values of 7 to 16 fetuses from at least 2 different litters of each genotype or 8 to 18 2-week-old mice (pooled for analysis in 2-6 replicates). *P < .01 comparing Il7r^−/− and Fl^−/−Il7r^−/− mice; **P < .01 comparing WT and Fl^−/− mice.