The AAV2 transgene plasmid in use in an ongoing choroideremia gene therapy phase 1/2 clinical trial (NCT01461213) was previously designed to express REP1 in the most efficient way: it contains a ubiquitous CAG promoter, Kozak consensus sequence, WPRE sequence, and a bovine growth hormone polyadenylation signal (pAAV2-CAG-REP1-WPRE-pA).13, 31 To investigate the effects of the WPRE in cell transduction in vitro and in vivo, the original plasmid was modified to express GFP by excision of human CHM cDNA, with and without WPRE: pAAV2-CAG-GFP-WPRE-pA and pAAV2-CAG-GFP-pA (Figure 1A). Here, CHM is linked to choroideremia.