Neomycin phosphotransferase II confers resistance to kanamycin and neomycin in bacteria and G418 (Geneticin? G418 Sulfate) in mammalian cells. These antibiotics bind to ribosomal components and inhibit protein synthesis. NPT II inhibits these antibiotics through phosphorylation, which is thought to interfere with their active transport into the cell. Immunoblot analysis of transfected cell lysates with anti NPT II can be used to gauge relative transfection efficiencies in experiments where multiple transfections are performed in parallel with different plasmids.