Neomycin phosphotransferase II (nptII) gene is used in selection of transformed organisms. It was initially isolated from the transposon Tn5 that was present in the bacterium strain Escherichia coli K12. The gene codes for the aminoglycoside 3'-phosphotransferase (denoted aph(3')-II or NPTII) enzyme. NPTII is probably the most widely used selectable marker for plant transformation. It is also used in gene expression and regulation studies in different organisms in part because N-terminal fusions can be constructed that retain enzymatic activity. In animal cells, G418 and neomycin are used as selectable agents.