Ku DEB activity in CLL extracts as determined by EMSA. Protein extracts (0.5 μg) obtained from purified B-CLL cells were incubated with ³²P-labeled 25-bp DNA probe in the presence of 1 μg of closed circular plasmid DNA, as a nonspecific competitor. The electrophoretic mobility of the protein-DNA complexes were analyzed in a 12% polyacrylamide gel as described in Materials and Methods. The positions of protein-DNA complexes consisting of DNA-end binding activity are indicated (H, heterodimer that corresponds to full-length Ku 70/full-length Ku 86 subunits; L, heterodimer that corresponds to full-length Ku 70/variant Ku 86 subunits). U, untreated CLL lymphocytes; T, treated-resistant CLL lymphocytes; C, MO59K control cells.