An expression system combining a unit for the expression of the gene o
f interest reinforced by the hepatitis B virus X transactivator and a
selectable gene weakened by the insertion of an A+T-rich sequence deri
ved from the 3'-untranslated region of the granulocyte-macrophage colo
ny-stimulating factor mRNA is described. This Vector allows rapid one-
step isolation of highly productive Chinese hamster ovary clones.