It is essential to select only those host organisms that have been transformed and that contain a properly constructed vector. In the case of plasmid transfor- mation, selection can be accomplished through the use of antibiotics and/or chromogenic (color-producing) substances. For example, the lacZ gene in the pUC18 plasmid (see Fig. 3-22) encodes the enzyme ␤-galactosidase, which cleaves the colorless compound X-gal to a blue product: