ASD-2b promotes E10 inclusion by altering 5′ splice site usage in the intron downstream from the regulated exons. In embryos, E10 is removed in a fast splicing event, leaving a product with E9 and E11 ligated together as the predominant partially spliced isoform, which is processed to remove intron 8 in a slower reaction. In adults, ASD-2b, through an unknown mechanism, promotes usage of the E10 donor site in the fast reaction. This results in a competition between E9 and E10 acceptor sites. E9 is removed because it lies downstream from a weaker 3′ splice site.

Potential mechanisms for exon inclusion stimulated by AS regulators binding to the downstream intron. In the recruitment pathway, the intron-bound AS regulator promotes exon recognition through specific contacts with the splicing machinery. TIA1 recruitment of U1 snRNP to 5′ splice sites of exons that it regulates serves as an example. In the conformational pathway, regulators bound to an intronic site promote regulated exon inclusion by intron definition. This might involve an interaction between regulators bound at either end of the intron that would serve to loop out intronic sequences and bring the regulated exon into proximity with the downstream constitutive exon, thereby favoring its inclusion.