Tumor-associated thymic progenitors are enriched in disease initiating potential. Leukemic cells from Tal1/Lmo2 mice were stained with CD4, CD8 antibodies, and lineage-negative cells were stained with CD25 and CD44 antibodies. (A) Tal1/Lmo2 mouse T-ALL cells were left unsorted or sorted into DN3 or DP populations by flow cytometry. Serial dilutions of unsorted, purified DP, or DN3 cells were transplanted into syngeneic recipients and monitored for disease. Three independent Tal1/Lmo2 tumors were analyzed. (B) Purified DN3 cells recapitulate the leukemic immunophenotype. Leukemic cells were isolated from mice transplanted with unsorted or purified DN3 cells and stained with CD4, CD8 antibodies, and lineage-negative cells were stained with CD25 and CD44 antibodies and analyzed by flow cytometry. Three Tal1/Lmo2 mouse T-ALLs were reexamined; one representative tumor is shown.