In situ analysis of γ-radiation–induced apoptosis of splenocytes. Sections from untreated (top panels) or γ-radiated (bottom panels) spleens derived from wt (A-B), Puma-deficient (C-D), and Bim-deficient (E-F) animals 20 hours after exposure to 5.0 Gy 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. Analysis of sections failed to reveal TUNEL⁺ apoptotic cells in control animals (A,C,E). Numerous TUNEL⁺ apoptotic cells are visible in sections derived from wt animals (B), but significantly fewer green cells appear in Puma^-/- or Bim^-/- sections (C,E). Representative images of 3 or more independent stains performed on organs of at least 2 animals per genotype and treatment are shown.