CFTR exon 9 presents a 3′ splice site polymorphism, (UG)_mU_n, whose composition influences splicing. TDP43 specifically binds the UG tract of the transcript and inhibits splicing in vitro. We report that depletion of TDP43 through RNA interference removes splicing inhibition caused by unfavorable (UG)_mU_n sequences, indicating that TDP43 exerts a potent inhibitory effect in vivo. We also show that the UG–TDP43 interaction has a dominant role over other exon 9 splicing regulatory elements. These results suggest that TDP43 association near a splice site has determined the evolution of positive splicing regulatory elements to contrast this inhibition.