High Quality Content by WIKIPEDIA articles! A spliceosome is a complex of specialized RNA and protein subunits that removes introns from a transcribed pre-mRNA segment. This process is generally referred to as splicing. Each spliceosome is composed of five small nuclear RNA proteins, called snRNPs, and a range of non-snRNP associated protein factors. The snRNPs that make up the nuclear spliceosome are named U1, U2, U4, U5, and U6, and participate in several RNA-RNA and RNA-protein interactions. The RNA component of the snRNP is rich in uridine. The canonical assembly of the spliceosome occurs anew on each hnRNA. The hnRNA contains specific sequence elements that are recognized and utilized during spliceosome assembly. These include the 5' end splice, the branch point sequence, the polypyrimidine tract, and the 3' end splice site. The spliceosome catalyzes the removal of introns, and the ligation of the flanking exons.