Nuclear pre-mRNA splicing is catalyzed by the spliceosome. The spliceosome is composed of five small nuclear ribonucleoprotein particles (snRNPs), along with many associated protein cofactors, that assemble to catalyze the removal of introns from pre-mRNA and the joining of exons to produce mature mRNA. Each snRNP contain a specific snRNA (which base-pairs with pre-mRNA to direct spliceosome assembly and splicing), a common Sm protein core and a set of specific subunits. (Adapted from PMID:23118483).