Reduced genome-wide DNA damage levels by REL_DKK obligate heterodimer variant pair of FokI nuclease domain. A), Representative images of cells treated with the DNA cleavage agent etoposide or transfected with the indicated ZFN expression plasmids. Cells were fixed after 30 h and stained with antibodies against 53BP1 (red) and then with DAPI (blue). The fraction of cells containing more than 3 foci is indicated under each panel. The total number of cells analyzed is: etoposide, 152; FokI_WT, 245; EL_KK, 282; REL_DKK, 289. B), ZFN expression levels were examined by anti-FokI immunoblot analysis. HEK293 Flp-In cells were transfected with indicated ZFN expression plasmids and cells were harvested after 30 h. Equal amounts of total cellular protein was separated by 10% SDS-PAGE gel and transferred to PVDF membrane. The blot was probed with anti-FokI antibody. The ZFNs migrate as a single band on the gel. Western blot analysis shows comparable levels ZFN expression for various obligate heterodimer variants in HEK293 Flp-In cells.