Nucleic acid from which a polypeptide with utrophin function can be expressed, especially mini-genes and chimaeric constructs. Expression significantly decreases the severity of the dystrophic muscle phenotype in an animal model, indicating usefulness in treatment of muscular dystrophy. The nucleic acid and encoded polypeptides are also useful in screening for substances to modulate utrophin binding to actin and/or the dystrophin protein complex.