IL-7 up-regulates Bcl-2 in T-cell–committed progenitors in all 3 Tg lines and decreases their proliferation only at high IL-7 overexpression. Mice (6 weeks old) derived from all 3 Tg⁺ lines and normal control mice were studied for Bcl-2 (A) and Ki-67 (B) expression in DN1 to DN4 progenitors. Panels A-B show 1 of 4 representative experiments. Shaded histograms indicate the marker of interest; and open histograms, the negative isotype controls. Thymocyte progenitor proliferation was also determined by in vivo BrdU incorporation in 6- to 8-week-old TgA, TgB, and normal control mice (C). Panel C shows the percentage of BrdU⁺ cells in DN1 to DN4 in all 5 experiments. ♦ represents normal B6, represents TgA, and □ represents TgB mice.