To define the extent of intervening sequences required for efficient splicing of the CYH2 gene in Saccharomyces cerevisiae, we have constructed a series of intron mutations. Artificial intron extensions of more than 300 bp of the natural intron lead to an inhibition of splicing whereas intron deletions lead to a drastic improvement of the splicing efficiency. It is shown that deletion of a 32 bp sequence element within the intron is responsible for this drastic improvement.