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The pre-mRNA (precursor mRNA) plus assorted snRNPs assemble and disassemble a spliceosome, which carries out the splicing reaction. The various snRNPs are designated U1, U2, and so on. In step 1 U1 is bound, followed by U2 binding (step 2). Factor U4/U6 and U5 then bind (step 3) and cleavage and transfer then occur (steps 4 and 5). The spliceosome dissassembles, releasing the joined exons (6) and the excised intron as a lariat structure (7). The excised intron is ultimately degraded into oligonucleotides (step 8).