U2OS and HEK293 cells were cultured in Advanced DMEM (Life Technologies) supplemented with 10% FBS, 2 mM GlutaMax (Life Technologies), and penicillin/streptomycin at 37 °C with 5% CO2. U2OS cells (program DN-100) and HEK293 cells (program CM-137) were transfected in 20 μl Solution SE on a Lonza Nucleofector 4-D according to the manufacturer’s instructions. dsODN integration rates were assessed by restriction fragment length polymorphism (RFLP) assay using NdeI. Cleavage products were run and quantified by a Qiaxcel capillary electrophoresis instrument (Qiagen) as previously described32 (link).