Apoptotic susceptibility of IL-7Rα Tg thymocytes. TUNEL assay was performed on thymocytes that were cultured in vitro for 20 hours with or without 3 ng/mL rmIL-7. To measure DNA fragmentation, staining for dUTP (expressed as TUNEL-positive) is shown in gated DN cells (A) and DP cells (B) of wild-type (upper row) or B1 mice (lower row). Percentages of TUNEL-positive and TUNEL-negative cells are indicated. One representative experiment of 3 is shown.