Effects of Δ446 mutation on CD4 and CD8 expression during T-cell ontogeny. Embryonic thymi from wild-type (AML1^+/+), AML1^WT/WT, and AML1^Δ446/Δ446 genotypes at several developmental stages, including E16.5, E17.5, and E18.5, were analyzed for CD4 and CD8 expression by flow cytometric analysis. Δ446 mutation resulted in an increase in the CD4 SP cell population, as indicated. Total numbers of thymus cells are shown above the quadrant graphs. Double-negative populations at E18.5 were further analyzed by 4-color analysis for CD25 and CD44 expression, as shown in the panels on the right. Thymi from the Δ446 mutants showed no difference in DN-stage progression compared with those from controls.