Myotube cultures of the myogenic cell line, C2, produce significantly lower levels of dystrophin than primary mouse cultures. We demonstrate that expression of the C2 dystrophin gene increases 10-fold in hybrid myotubes formed by fusion of C2 and dystrophin-deficient human myoblasts from a Duchenne muscular dystrophy patient. These results indicate that C2 cells are deficient in endogenous gene regulatory factors which enhance dystrophin expression, and that the C2 cell line may therefore be used to identify putative trans-acting factors involved in the regulation of dystrophin gene expression.