The presence of a pseudouridine residue in U2 snRNA, nearly opposite of the branch site, results in an altered conformation of the RNA-RNA duplex upon the U2 snRNP binding. Specifically, the altered structure of the duplex induced by the pseudouridine places the 2' OH of the bulged adenosine in a favorable position for the first step of splicing. The U4/U5/U6 tri-snRNP (see Figure 1) is recruited to the assembling spliceosome to form complex B, and following several rearrangements, complex C is activated for catalysis. It is unclear how the tri-snRNP is recruited to complex A, but this process may be mediated through protein-protein interactions and/or base pairing interactions between U2 snRNA and U6 snRNA.