Expression of the dystrophin-related protein (DRP or Utrophin) was examined with Western blot and immunohistochemical methods in diaphragm, limb and also in myoblast transferred muscles of the mdx mouse. Although we have hypothesized that progressive fibrosis in the diaphragm of the mdx mouse has been due to a smaller amount of DRP expression compared with limb muscles, we could not find any difference in the amount of DRP or in the DRP localization pattern between the two muscle sites. In limb muscles treated with myoblast transfer, dystrophin-positive muscle fibers had no DRP on their surface membrane, although dystrophin-negative muscle fibers were DRP-positive. These findings suggest that excessive expression of DRP is suppressed in the normalized muscle fiber with dystrophin. It also appears that the histological differences seen in the different muscles of the mdx mouse are not due to the amount of DRP present.