The availability of the complete sequence of the genome of Saccharomyces cerevisiae (Goffeau et al., 1996) and other organisms has provided a substantial amount of information on their chromosomal organization. We need now to understand the coordinated expression of the many genes present in a given genome and the function of the various encoded proteins. Microarrays (DNA chips) offer the opportunity to analyze nucleic acids at the genomic scale and have been used to perform global gene expression studies (Lander, 1999). However, these techniques also have the potential to give us a new perspective on posttranscriptional processes involved in the regulation of gene expression. We provide here a striking example through the investigation of yeast pre-mRNA splicing. Our analysis demonstrates that pre-mRNA splicing is quantitatively a much more important process in this species than previously thought and shows that splicing-signal conservation is correlated with transcription efficiency.