A modified TOTO-1 iodide labeling protocol to enable visualization of nucleic acid loading efficiency, as a means to verify that nucleic acid constructs are entering the cells. With additional cleanup steps to decrease background staining. A convenient assay to use in conjunction with transfection optimization. (Modified from Golzio, M., Teissie, J. & Rols, M.P. 2002. Direct visualization at the single-cell level of electrically mediated gene delivery. Proc Natl Acad Sci USA 99, 1292–1297.)