Thymic progenitors are expanded in preleukemic Tal1/Lmo2 mice. (A) Thymocytes from 6-week-old wild-type or preleukemic Tal1/Lmo2 mice were stained with CD4-PE-Cy5 and CD8-PE or with a lineage cocktail consisting of CD4-PE, CD8-PE, B220-PE, GR1-PE, and MAC1-PE. Lineage-negative cells were then stained with CD44-allophycocyanin and CD25-PE-Cy7 and analyzed by flow cytometry. (B) Decreased thymic cellularity in preleukemic Tal1/Lmo2 mice. Thymic cellularity was determined using trypan blue exclusion. (C) The absolute number of DN3 and DN4 progenitors is increased in Tal1/Lmo2 mice. The absolute number in each thymic subset was determined by multiplying the percentage of each thymic subset by the total thymic cellularity.