Structure and function of the spliceosome When RNA is transcribed from DNA in the eukaryotic cell nucleus, the initial transcript includes noncoding introns that must be spliced out. This splicing is done by a complex macromolecular machine, the spliceosome, which comprises five small nuclear RNAs and more than 100 associated proteins. Now, two papers reveal insights into the structure and function of the yeast spliceosome. Yan et al. describe a high-resolution structure determined by electron microscopy of a spliceosome complex comprising four RNAs and 37 proteins. Hang et al. focus on the catalytic site and show how protein components anchor the transcribed RNA and allow sufficient flexibility to deliver RNA components involved in catalyzing the splicing reaction. Science , this issue pp. 1182 and 1191