pre-mRNA splicing mRNA Splicing GENE ONTOLOGYGO:0008380 Pubmed12226669 Reactome Database ID Release 4372172 Reactome, http://www.reactome.org ReactomeREACT_1735 The process in which excision of introns from the primary transcript of messenger RNA (mRNA) is followed by ligation of the two exon termini exposed by removal of each intron, is called mRNA splicing. Most of the mRNA is spliced by the major pathway, involving the U1, U2, U4, U5 and U6 snRNPs. A minor fraction, about 1 %, of the mRNAs are spliced via the U12 dependent pathway. U12 Dependent Splicing PathwayStep47 PathwayStep41