The invention provides a method for enhancing expression of a transgene in a host cell, which includes the steps of inserting a capsid promoter element (Pcap), or a sequence comprising the reverse complement of the capsid promoter element (PcapR), into a mammalian expression cassette upstream (5′) of a cytomegalovirus immediate/early enhancer/promoter region (Pcmv); inserting the transgene into the expression cassette downstream (3′) of the cytomegalovirus immediate/early enhancer/promoter region; inserting a vector containing the expression cassette into the host organism; and causing expression of the transgene. The capsid promoter element (Pcap) is typically from a circovirus, parvovirus or anellovirus. The transgene is typically expressed at a higher level than when expressed by a vector containing the expression cassette without the transcriptional control element. An expression cassette, vector, DNA vaccine, pharmaceutical composition and method of treatment are also claimed.