Gadd45β blocks caspase activation by Fas. (A) Western blots showing caspase processing in Neo, HA-Gadd45β, and DN-FADD BJAB clones treated with anti–APO-1 for the time shown. Antibodies, molecular weights of specific proteins and cleavage products, and nonspecific (n.s.) bands are indicated. High molecular weight products detected with the caspase-3–specific antibody (*) likely represent previously described ubiquitinated polypeptides. (B) In vitro fluorimetric assays monitoring caspase-8, -9, and -3/7 activity in select samples of the same extracts used in (A). Values express fluorescence units after subtraction of the background values obtained with unstimulated cells and represent the mean ± standard deviation of 3 independent experiments.