This transgene contains the tetracycline responsive hCMV*-1 promoter [consisting of the tetracycline responsive element (TRE) and a minimal hCMV promoter] [25] from pTRE2 (Clontech Inc., Mt. View, CA), a chimeric intron from pCI (Promega) to increase message stability and expression [26,27], a Kozak consensus sequence at the initiation codon to optimize translation [28], the human E2 cDNA which has been modified to contain a 4× alanine linker followed by a c-myc epitope tag at the carboxy terminus to facilitate detection, and an SV40 late polyadenylation sequence from pCI for enhanced mRNA stability and translation.