In vitro NHEJ assays were performed essentially as described previously (19 (link)). Briefly, reactions (10 μl) were performed in 50 mM HEPES pH 8.0, 100 mM KOAc, 0.5 mM Mg(OAc)2, 1 mM ATP, 1 mM DTT and 0.1 mg/ml bovine serum albumin (BSA) and the indicated concentration of Ku-DBi. pBluescript DNA was linearized with HindIII and 5′-32P-labeled. Reactions contained 10 ng of DNA and 25 μg of whole cell extract. Reactions were incubated at 37°C for 2 h, terminated by the addition of proteinase K/SDS/EDTA and incubated for an additional 30 min at 37°C. Products were separated by electrophoresis on 0.6% agarose gels. Products were detected in dried gels by PhosphorImager (BioRad PMI) analysis.