Overexpression of FOXO protein promotes apoptosis. Transformed pro/pre-B cells were retrovirally infected with pMIT, FOXO3a, or FOXO3a(A3) and analyzed by FACS 48 hours after infection. The extent of apoptosis in cells with different levels of Thy1.1 expression was determined by Annexin V staining. The numbers in the upper-right quadrant represent the fraction of Thy1.1-positive cells that is Annexin V-positive. Data are representative of 3 experiments and 1 to 2 cell lines per genotype.