The quality and the concentration of DNA used for Nucleofection™ in general plays a central role for the efficiency of gene transfer. We strongly recommend using endotoxin free prepared DNA. Endotoxin free Kits are available from several suppliers. The presence of endotoxins can increase cell mortality and this is especially true of sensitive cells such as primary neurons. The presence of endotoxins can increase cell mortality and this especially true of primary cells such as neurons. The purified DNA should be resuspended in deionized water or TE buffer (10 mM Tris/HCl, 1 mM EDTA, pH 8.0) with a concentration of 1-5 mg/ml. We find that 1mg/ml is ideal. Please check the purity of each plasmid preparation by measuring the A260:A280 ratio.