Disrupting heterodimerization with CBFβ substantially abrogates TEL-AML1's ability to enhance the clonogenic potential of c-kit⁺ B-cell precursors. (A) Detection of hemagglutinin (HA)–tagged TEL-AML1 and the T188A and Y140A/T188A mutants in NIH3T3 cells 48 hours after retroviral (murine stem cell virus [MSCV]) transduction. Vertical lines have been inserted to indicate a repositioned gel lane. (B) Flow cytometric analysis of EGFP expression in c-kit⁺ fetal liver cells 48 hours after infection with viral supernatants. (C) Transduced c-kit⁺ cells grown in methylcellulose serial replating assays in conditions promoting B-cell differentiation (IL-7, SCF, and FLT3 ligand). The number of colonies harvested from each culture, per 10⁴ cells plated, is shown for the first (□), second (), and third (■) rounds of plating. Plots show means and SDs of duplicate cultures. The data shown are representative of 5 independent experiments. (D) Total number of cells, as in panel C. (E) Number of B220⁺ cells as in panel C.