Splicing is the process in which pre-mRNA is modified by the removal of introns to form mRNA, comprising only joined exons. These introns normally exist within protein-coding sequences within the genome. Splicing is neccessary in many eukaryotic cells for transcription, in order for them to form functioning proteins by the secondary process of translation of the cell's mRNA. Typically, splicing is achieved through a series of reactions, catalyzed by a spliceosome, a large complex of five small nuclear ribonucleoproteins (snRNPs), however self-splicing introns do exist for introns that form ribozymes, where the function of the spliceosome is performed by the RNA alone. Although splicing exists throughout the kingdoms of life, the extent and type difers greatly. For example, although common in many eukaryotic organisms, splicing occurs rarely in prokaryotes and lacks a spliceosomal pathway.