The present invention relates to transgene expression systems, related pharmaceutical compositions, and methods of making and using them. Preferred systems employ an adenovirus transgene expression vector comprising DNA sequence encoding a transgene which codes for a desired product, expressibly contained within an adenovirus vector containing at least a portion of the E3 region and certain portions of the E4 region. The E4 portions comprise the open reading frame sequence known as E4ORF3 and at least one other portion of E4. Preferably the E4 portion of the vector (or "E4 cassette") includes E4ORF3 and at least one other portion selected from E4ORF4, E4ORF6/7 and E4ORF3/4. The invention has a number of important features including improving persistency of transgene expression in a desired host cell. The transgene expression systems of the present invention are useful for a variety of applications including providing persistent cellular expression of the transgene in vitro and in vivo.