2.3. Homologous Recombination Assay

HR frequency was measured by stable or transient transfection of I-Sce-1-linearized pDR-GFP (Addgene #26475) and DR-oriP-GFP (kindly provided by M. Jasin) plasmids. Briefly, 1 µg linearized plasmid (pDR-GFP) or 0.5 µg (DR-oriP-GFP) linearized plasmid plus 0.5 µg MSCV-N-EBNA1 (Addgene #37954) was transfected into cells using FuGENE (Roche, Basel, Switzerland) in a 1:3 µg/µL ratio according to the manufacturer’s instruction. After 24 h cells were harvested, and the fraction of GFP-positive cells was determined by flow cytometry.