There are only about 250-300 introns in the native S. cerevisiae genome. Numerous individual introns have been precisely removed at the DNA level without profound effects on mitotic growth, although effects on meiosis have been noted in some cases. The major exception to this observation is that deletion of introns encoded within certain ribosomal protein genes has been shown to yield fitness defects. Thus, only those introns whose deletion is not associated with fitness defects will be removed; all ribosomal protein gene introns will be retained, at least for the time being. Certain introns contains snoRNAs that may be required for fast growth. In cases where introns are shown to encode snoRNAs or other important small RNA molecules, we will relocate these to non-intronic positions (most snoRNA genes are already non-intronic in S. cerevisiae). Large introns are excellent candidates for homes of yet-to-be-found classes of small RNAs in the yeast genome. The contribution of RNA molecules to regulation of biological complexity in eukaryotes may well have been vastly underestimated, and certainly recent studies of the RNA genomics of metazoans lead credence to this hypothesis. But to what extent does this apply to the relatively streamlined yeast genome? We will be providing evidence for or against this hypothesis depending on the outcome of our intron deletion experiments. If we can continue to delete them with impunity and without obvious impact on fitness (for example, making an entirely intron-free chromosome), it will suggest that there are few essential RNAs imbedded in introns. But if we find specific phenotypic effects of deleting specific introns, this will be a very exciting result, requiring follow-up. Such results would imply either a previously unknown and critical RNA product or a previously unknown essential role for splicing. These can be dissected by expressing the intronic sequences ectopically; if such constructs “complement” intron deletion phenotypes it is strong presumptive evidence for an intron-encoded product.