Introns stimulate gene expression in a wide range of organisms by increasing the levels of mature mRNA， without affecting mRNA stability. Although introns sometimes function as transcriptional enhancers， they usually stimulate expression by a process termed intron-mediated enhancement (IME). The mechanism of IME is largely unknown. While splicing per se is not sufficient for IME， as evident from the fact that not all introns increase expression， it is not clear yet whether splicing of the enhancing introns is essential for enhancement. The leader intron (LI) of the Arabidopsis AtMHX gene was previously shown to substantially increase the expression of the AtMHX promoter. Here we investigated whether this LI acts as a transcriptional enhancer and whether its splicing is essential for IME.