a) U2OS ER-AsiSI cells were transfected with a siRNA against CtIP. 52 hours following transfection, total RNA was prepared, retrotranscribed using random primers and levels of CtIP cDNA relative to ribosomal phosphoprotein P0 were assessed by qPCR. b) Same as in a), except that 4OHT was added 48h following transfection to induce AsiSI digestion as described in ref 13. 4 and 24 hours later, as indicated, genomic DNA was prepared and the amounts of AsiSI-cohesive breaks were quantified at two AsiSI sites by Ligation-Mediated Purification. Note that DNA resection leads to the underestimation of the presence of breaks 24 hours, but not 4 hours, following break induction.