Since the identification in 1987 of the gene for Duchenne muscular dystrophy (DMD), research on the molecular pathogenesis of muscular dystrophy has progressed extensively. In particular, discovery of the DMD gene product, dystrophin, led to the identification of dystrophin-associated proteins and, subsequently, the recognition of other types of muscular dystrophy caused by the defects in each of the sarcoglycan genes. On the other hand, effective therapy for DMD has not yet been established. Some of the viral vectors, such as adeno-associated virus vectors or lentiviral vector, have been proven to enable the long-term expression of the exogenous gene without overt host immune reactions. However, dystrophin cDNAs are too large (14kb) to be accommodated in these viral vectors. To solve this problem, we and other research groups succeeded in truncating full-length dystrophin cDNA to small dystrophin cDNA (4 to 5kb), the products of which protect dystrophin-deficient mdx muscle from contraction-induced membrane damage when introduced by viral vectors or as a transgene into mdx mice. The usefulness of these truncated dystrophin cDNAs should be confirmed using other animal models such as dystrophic dogs. To develop successful treatment of DMD, the authors believe that several different approaches should be used, such as cell transfer therapy, drug design to up-regulate utrophin, or a strategy to repair the mutation in vivo.