Abstract: The present invention relates to transgene expression systems, related compositions comprising the transgene expression systems, and methods of making and using them. Preferred systems employ an adenovirus transgene expression vector comprising DNA encoding a transgene which codes for a desired product operably linked to expression control sequence, and at least a portion of the adenovirus E3 region and certain portions of the E4 region. The E4 portions comprise the open reading frame sequence known as E40RF3 and at least one other portion of E4. Preferably the E4 portion of the vector (or “E4 cassette”) includes E40RF3 and at least one other portion selected from E40RF4, E40RF6/7 and E40RF3/4. The invention has a number of important features including improving persistency of transgene expression in a desired host cell.