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. RNA-binding protein that binds to 5'ACACCC-3' core sequence, termed zipcode, within the 3'UTR of ITGA3. Binds to CUG triplet repeat expansion in myotonic dystrophy muscle cells by sequestering the target RNAs. Seems to regulate expression and localization of ITGA3 by transporting it from the nucleus to cytoplasm at adhesion plaques. May play a role in myotonic dystrophy pathophysiology (DM). Interacts with ITGA3. Greater concentration in the nucleus. Expressed in or near large cytoplasmic adhesion plaques (PubMed:16273094). Location in the cytoplasm is microtubule-dependent (PubMed:16273094). In both DM1 and DM2 patients, colocalizes with nuclear foci of retained expanded-repeat transcripts (PubMed:11929853). Expressed in heart, brain, placenta, lung, liver, skeletal muscle, kidney and pancreas. Belongs to the muscleblind family.