Downstream of the CMV enhancer/promoter region is a chimeric intron composed
of the 5´-donor site from the first intron of the human β-globin gene and
the branch and 3´-acceptor site from the intron of an immunoglobulin gene
heavy chain variable region (7). The sequences of the donor and acceptor sites,
along with the branchpoint site, have been changed to match the consensus
sequences for splicing (8). The intron is located upstream of the cDNA insert in
order to prevent utilization of possible cryptic 5´-donor splice sites within the
cDNA sequence (9).