Notch1-active thymic progenitors are expanded in preleukemic Tal1/Lmo2 mice. (A) Six- to 8-week-old TNR/+ or Tal1/Lmo2/TNR mice were killed and thymocytes stained with CD4, CD8 antibodies. DP, SP, and DN populations were analyzed for percentage GFP⁺ cells by flow cytometry. (B) Lineage-negative cells were stained with CD25 and CD44 antibodies, and GFP⁺ was analyzed by flow cytometry. The percentage of GFP⁺ cells in the DN1, DN2, DN3, and DN4 progenitor populations is shown. (C) Using specific primers for Vβ1-18, TCR Vβ mRNA expression was examined in wild-type and sorted GFP⁺ DN3 and 4 thymic progenitors isolated from preleukemic Tal1/Lmo2 mice. Three mice were examined; one representative experiment is shown. Vertical lines have been inserted to indicate repositioned gel lanes.