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 (By similarity). Could inhibit terminal muscle differentiation, acting at approximately the time of myogenin induction. Mostly nuclear. High expression in proliferating myoblasts is strongly reduced in differentiated muscle cells. Belongs to the muscleblind family.