Intron-mediated enhancement has been documented in many cases to involve large positive effect on gene expression. To address this, human Alpha-1 antitrypsin (hAAT) gene was integrated into Pichia pastoris with and without a yeast intron generated from the final plasmid pBlu-exII-int-exIII and ligated into the EcoRI/BamHI multiple cloning site of the yeast shuttle vector pHIL-S1. The chimeric exon-intron complex in the middle of the naturally occurring hAAT exons II and III caused a 23-fold enhancement of hAAT expression in P. pastoris, measured through SDS-PAGE and immunoblot analyses.