In the first step, the 2'-OH of an adenosine in the intron (the branch site A) attacks the phosphodiester bond at the 5' splice site to release the upstream exon and form a branched or lariat ...

To explore the evolution and function of introns in eukaryotes, we have developed an intron sequence information ... with cryptic exons, 5′ exon extensions and 3′ exon extensions occurring ...