RNA splicing is a process in molecular biology where a newly-made precursor messenger RNA (pre-mRNA) transcript is transformed into a mature messenger RNA (mRNA). It works by removing all the introns (non-coding regions of RNA) and splicing back together exons (coding regions). For nuclear-encoded genes, splicing occurs in the nucleus either during or immediately after transcription. For those eukaryotic genes that contain introns, splicing is usually needed to create an mRNA molecule that can be translated into protein. For many eukaryotic introns, splicing occurs in a series of reactions which are catalyzed by the spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs). There exist self-splicing introns, that is, ribozymes that can catalyze their own excision from their parent RNA molecule. The process of transcription, splicing and translation is called gene expression, the central dogma of molecular biology.