Transgene expression in maize cells changed from intron-independent to intron-dependent by an exact exchange of the bar coding region for that of cat. By deletion mapping an approximately 100 nucleotide sequence element at the 5′ end of the cat coding region was identified that, when inserted at the translation start site of the bar gene, impaired expression. Successive inclusion of the salT intron in the 5′ untranslated region (UTR) restored expression near to wild-type bar expression levels. A chimeric gfp gene, but not nptII gene, behaved similarly. These observations are in agreement with the view that intron-mediated enhancement of transgene expression does not enhance, but rather restores expression of an impaired gene.