In molecular biology, splicing is an editing of the nascent pre-messenger RNA (pre-mRNA) transcript in which introns are removed and exons are joined together (ligated). For nuclear-encoded genes, splicing takes place within the nucleus after or concurrently with transcription. For those eukaryotic genes that contain introns, splicing is usually required in order to create an mRNA molecule that can undergo translation, a process that creates proteins. For many eukaryotic introns, splicing is carried out in a series of reactions which are catalyzed by the spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs). Self-splicing introns also exist.