Mediates pre-mRNA alternative splicing regulation. Acts either as activator or repressor of splicing on specific pre-mRNA targets. Inhibits cardiac troponin-T (TNNT2) pre-mRNA exon inclusion but induces insulin receptor (IR) pre-mRNA exon inclusion in muscle. Antagonizes the alternative splicing activity pattern of CELF proteins. May play a role in myotonic dystrophy pathophysiology (DM). Could inhibit terminal muscle differentiation, acting at approximately the time of myogenin induction. {ECO:0000269|PubMed:12297108, ECO:0000269|PubMed:15257297}. This is the function of MBNL3 (muscleblind like splicing regulator 3, ENSG00000076770).