γ-Radiation–induced apoptosis of thymocytes is mediated by a Puma- and Bim-dependent mechanism. Thymic sections derived from wt (A-C), Puma-deficient (D-F), and Bim-deficient (G-I) animals 20 hours after exposure to 2.5 Gy γ-radiation were TUNEL stained using FITC-dUTP to detect nicked DNA in apoptotic cells. Nuclei were counterstained using 7-AAD. Sections were analyzed using a ZEISS Axiovert fluorescence microscope. The top panels show stains of representative thymic sections from untreated animals, revealing comparable numbers of TUNEL⁺ (green) cells in all genotypes analyzed. After exposure to 2.5 Gy γ-radiation, marginal zone and cortical structures are no longer visible in the wt (B) and are reduced in Bim^-/- (H) thymic sections but clearly maintained in the Puma^-/- thymus (E). At higher magnification (bottom panels), sections from wt mice (C) exhibit significantly more TUNEL⁺ apoptotic cells than those from Puma^-/- or Bim^-/- animals (F,I). The images are representative of 3 or more independent stains performed on organs of at least 2 animals per genotype and treatment.