A high level of K19/CAT fusion mRNA was detected when this K19 3' enhancer sequence was present at the 3' end of the fusion gene whereas no K19/CAT fusion transcript was detected if this 3' K19 enhancer sequence was absent, suggesting that the 3' K19 enhancer is crucial for the positive regulation of K19 expression.     ///    Co-transfections of a c-jun expression vector with K19/CAT reporter constructs demonstrated that c-jun was able to activate the K19 promoter via the 3' K19 enhancer.     ///    "Through transient transfection assays with various K19/CAT constructs that contain different portions of K19 genomic DNA, an enhancer sequence has been identified in the K19 3'-flanking region. "