Removal of non-protein coding introns from pre-mRNA involves a dynamic, high molecular weight complex known as the spliceosome. This nucleo-protein assemblage is comprised of five uridine rich small nuclear RNAs (snRNAs) as well as over 200 splicing proteins. All small nuclear ribonucleoprotein particles (snRNPs) (U1, U2, U4/U6 di-snRNP and U5) are labeled as they appear. E1 and E2 represent exons, separated by an intron (line) containing the branch point sequence (symbolized by an A). The names of each complex (E, A, B1, B2, C1, C2 and I) are designated in bold. ATP dependent reactions are indicated along the cycle. The degrading intron is symbolized by a dashed line.