Downstream-first model for ordered splicing of introns flanking exon 16.

The first step of the pathway is excision of the downstream intron, generating a partially spliced RNA with exons 16 and 17 joined. This step is hypothesized to remove repressor elements in intron 16 and/or juxtapose enhancer elements in exon 17, thus activating splicing of the upstream intron in the second step of the reaction.