Proliferation block in DN thymocytes of dnNb TG mice. (A) Maturation of WT and dnNb TG fetal (15 days) thymocytes to the DP stage after 6 days of coculture with OP9 or OP9-Δ-like cells. The DP stage was monitored by expression of CD4 and CD8. (B) Differentiation of WT and dnNb TG fetal DN thymocytes after 6 days of coculture with OP9 or OP9-Δ-like. Lin⁻ thymocytes were stained with CD44 and CD25. Quadrants represent DN1, DN2, DN3, and DN4 subpopulations (clockwise from top left to bottom left). (C) CFSE expression in fetal thymocytes of WT (solid line) or dnNb TG (broken line) mice cultured for 6 days over OP9-Δ-like cells. (D) Confocal images of fetal thymocytes of WT (top) and dnNb TG (bottom) mice stained with Numb (green), dnNb (anti-Myc; red) and CD25 (blue). Data are representative of at least 3 independent experiments.