Duchenne muscular dystrophy (DMD), the most common and severe form of the muscular dystrophies, is an X-linked inborn error of metabolism with multiple tissue involvement. Although the major pathological changes are observed in skeletal muscle, abnormalities have also been detected in the heart, nervous system, red blood cells, lymphocytes and cultured skin fibroblasts. For many reasons, such as readily available tissue material, fewer secondary changes and the potential for prenatal diagnosis, cultured skin fibroblasts should be the tissue of choice to search for the primary defect. Several abnormalities have been reported in DMD fibroblasts, suggesting that the genetic abnormality is expressed in these cells. To search for potentially mutant protein(s) we have compared the protein composition of normal and DMD fibroblasts by two-dimensional gel electrophoresis and have now found one protein spot consistently missing in DMD cells. The nature of this protein and its relation to the DMD gene are unknown.