The U937 cells are easy to transfect with the Celetrix electroporation system. Follow the suggested parameters and a high transfection efficiency would be achieved. Plasmids tend to cause cell death after electroporation in U937 cells. mRNA or protein (Cas9 RNP) does not cause cell death after electroporation. When using plasmids to generate a new engineered cell line, it is helpful to use a cell sorter to pick surviving cells from the transfected pool. The dying cells have a very distinct FSC/SSC profile and it is easy to collect surviving cells from the transfected pool.