FIG. 4.

Deletions of ISS sequence from the 5′ end uncover the existence of two silencer elements, ISS1 and ISS2. (A) A minigene that lacked ISAR was used as the backbone for a series of deletion mutants denoted Δ1 through Δ4. The relative positions of the two subelements are denoted as demonstrated in the lower panel. (B) Transfection of minigenes with progressive shortening of ISS reveals two regions with most of the splicing silencing effect. The percentage of IIIb inclusion is represented at the bottom. Sequences deleted in the Δ2 deletion mutant as well the sequences between Δ4 and −ISS displayed the most profound increase in exon IIIb inclusion.