Base pairing interactions between pre-mRNA and snRNAs (see also Figure 28.31 on page 1097). (A) Interactions between pre-mRNA and U1 snRNA and U2 snRNA. Note that interactions between U2 snRNA and pre-mRNA result in "bulging" the branch point A. (B) Base pairing between U4 and U6 snRNAs. (C) U6 snRNA later exchanges base pairing with U4 snRNA to U2 snRNA. RNA helicases facilitate these rearrangements. Base pairing with U5 snRNA also coordinates the 5'-exon and the 3'-exon prior to lariat formation.