ARF increases 6-4PP and CPD removal. (A) 6-4PP and CPD removal rates were compared between DKO, TKO, WT and ARF−/− MEFs, and (B) in DKO cells in which ARF was knocked down by the expression of an ARF shRNA (see supplementary information online for methods). Percentages of 6-4PP and CPD removed in three experiments were quantified and plotted. 6-4PP, 6-4 pyrimidine pyrimidone; ARF, alternative reading frame; Cont, control; CPD, cyclobutane pyrimidine dimer; DKO, double knockout; MEF, mouse embryonic fibroblast; shRNA, short-hairpin RNA; TKO, triple knockout; WT, wild type.