The level of resection adjacent to specific DSBs was measured by quantitative polymerase chain reaction (qPCR) using a modification of the original yeast method (23 (link)). The sequences of qPCR primers and probes are shown in Supplementary Table S2. Twenty microliters of genomic DNA sample (∼140 ng in 1× NEB restriction enzyme buffer 4) was digested or mock digested with 20 units of restriction enzymes (BsrGI, BamHI-HF or HindIII-HF; New England Biolabs) at 37°C overnight. Three microliters of digested or mock-digested samples (∼20 ng) were used as templates in 25 μl of qPCR reaction containing 12.5 μl of 2× Taqman Universal PCR Master Mix (ABI), 0.5 μM of each primer and 0.2 μM probe using a ViiA™ 7 Real-Time PCR System (ABI). The percentage of ssDNA (ssDNA%) generated by resection at selected sites was determined as previously described (24 (link)). Briefly, for each sample, a △Ct was calculated by subtracting the Ct value of the mock-digested sample from the Ct value of the digested sample. The ssDNA% was calculated with the following equation: ssDNA% = 1/(2^(△Ct-1) + 0.5)*100 (23 (link)).