The spliceosome is a large ribonucleoprotein that splices precursor messenger RNA (pre-mRNA) in the nuclei of eukaryotic cells (reviewed inrefs.1 –3 ). It is composed of five small nuclear ribonucleoproteins (snRNPs) and numerous non-snRNP proteins. Each snRNP consists of a small nuclear RNA (snRNA) and several proteins.
