Schematic representation of precursor mRNA (pre-mRNA) splicing. Top: Human genes have coding (exon) and noncoding (intron) sequences and are initially expressed as pre-mRNA that includes all of these sequences. In the process of precursor mRNA splicing or maturation, introns are excised and flanking exons are joined into the mature sequence of RNA. Bottom: Excision and ligation processes of pre-mRNA splicing are catalyzed by the spliceosome, a macromolecular structure composed of 5 snRNAs associated with proteins to form particles termed snRNPs. Each U1, U2, and U5 snRNP contains a single snRNA and several proteins, whereas U4 and U6 snRNPs contains 2 snRNAs and several proteins. In most eukaryotes, there are 2 classes of introns: the common U2 type (representing more than 99% of human introns) and the rare U12 type; these 2 types differ in their consensus splice-site sequences (indicated in black at the 5′ and 3′ end of the intron). The removal of these 2 types of introns is catalyzed by 2 different spliceosomes, called U2- and U12-dependent spliceosomes, respectively. The 2 spliceosomes differ in their snRNA composition, whereas most of the proteins are shared. The assembly of the spliceosome occurs anew on each pre-mRNA, which contains specific sequence elements that drive this process. The crucial signal sequences are the splice donor site (5′ end of the intron), the branch site (near the 3′ end), and the splice acceptor site (3′ end). In the major U2-dependent spliceosome, the 5′ donor site is recognized by the U1 snRNP, whereas the branch site is recognized by the U2 snRNP.