Alternative end-joining pathway is efficient but slow for both compatible and non-compatible ends. Ten to thirteen independent clones of CHOK1 and xrs5 cells harboring either pEJ (A) or pEJ2 (B) were transfected with the I-SceI-expressing vector. After a 24-h repair interval the fraction of GFP-positive/total cells (%GFP+) was measured by flow cytometry. Hatched columns represent mean values (±SE). xrs5-pEJ clone #11 was not included as deviation from others was larger than 2σ. NHEJ efficiency for both ends and was significantly lower in xrs5 than in CHOK1 cells (Mann–Whitney, P = 0.001). (C) NHEJ kinetics for non-cohesive (left panel) and cohesive (right panel) ends. Cells were harvested at different time intervals after transfection and analyzed for GFP+. The mean (±SE) values of three to four representative clones of each variant (CHOK1-pEJ #3, 40, 75; CHOK1-pEJ #33, 39, 48; xrs5pEJ #20, 23, 26, 59; and xrs5-pEJ2 #9, 10, 12) are shown. Differences between the two strains were significant only after 24 h (Mann–Whitney, P = 0.0025).