Addition of the woodchuck hepatitis virus posttranscriptional regulatory element (WPRE) to the NSE and PPE constructs improved expression levels by up to a further 10-fold in vitro and in vivo. WPRE is a DNA sequence that, when transcribed, creates a tertiary structure enhancing transgene expression. Insertion of the WPRE in the 3′ untranslated region of coding sequences carried by either oncoretroviral or lentiviral vectors substantially increased their levels of expression in a transgene, promoter, and vector-independent manner. Using adenovirus vectors, Xu et al. found that the WPRE in the sense orientation cloned between the luciferase gene and the polyA sequence stimulated 2- to 7-fold more luciferase expression in vitro and 2- to 50-fold more in the liver, kidney, and lung of mouse than occurred without the use of the WPRE . The most efficient Ad vector in this study, which contained an improved CMV promoter and the WPRE, showed more than 700-fold luciferase expression in mouse liver than did the Ad vector containing the conventional CMV promoter but no WPRE. These results indicate that inclusion of the WPRE, combined with the optimization of transcriptional regulatory elements in Ad vectors, can permit a given therapeutic goal to be achieved with substantially fewer viral particles. This and other studies utilizing WPRE provided novel information that incorporated this element in CNS gene therapy projects.