Splicing mechanism used for mRNA precursors. The upstream (5′) exon is shown in blue, the downstream (3′) exon in green, and the branch site in yellow. Y stands for a pyrimidine nucleotide, R for a purine nucleotide, and N for any nucleotide. The 5′ splice site is attacked by the 2′OH group of the branch-site adenosine residue. The 3′ splice site is attacked by the newly formed 3′OH group of the upstream exon. The exons are joined, and the intron is released in the form of a lariat.

[Information from P. A. Sharp, Cell 42:397–408, 1985.]