Immuno-slot-blot analysis was performed as described previously61. Briefly, cells (70% confluent) were exposed to 30 J/m2 UV and harvested immediately or indicated times post-treatment. Cells were lysed with 10 mM Tris pH 8.0, 1 mM EDTA, 0.05% SDS, 100 μg/ml fresh proteinase K) and DNA isolated. DNA was bound to a nitrocellulose membrane using a slot blot apparatus and probed using antibody for 6-4 photoproducts (6-4 PP; Cosmobio). Data are reported as percent repair compared to the amount of initial damage (0 hr time point).