Duchenne muscular dystrophy is a very severe disease which affects the skeletal musculature in human patients. It produces muscle wasting. Necrotic fibres are partially replaced by regeneration from dormant satellite cells, so that the disorder progresses slowly over the first two decades of life. Eventually, the degenerative process takes over, muscle tissue is replaced by fibrous material and fat infiltration, with severe deformations of the skeleton (spine) due to imbalance of muscle tone. Failure of the respiratory muscle is inevitably fatal. So far, no curative treatment is available. The disease affects only boys with a frequency of 1 in 3500 male births. It is one of the most frequent genetic diseases in the human race.

The origin of the disease was elucidated in 1987 when it was shown that it is due to mutation (s) affecting a very large gene located in the p21 region of the X chromosome, and coding for a protein termeddystrophin'(Hoffman et al., 1987a; Koenig et al., 1987). Mutations resulting in the absence of dystrophin are responsible for the very severe Duchenne phenotypes, while in milder diseases, called Becker dystrophy, dystrophin is expressed in low levels or in some truncated forms (Monaco et al., 1988; England et al., 1990). Duchenne-like dystrophies of various degrees of gravity have been observed in animal (mice, cats, dogs) and also result from mutations in the dystrophin gene. For further considerations of the relationship between mutations of the dystrophin gene and dystrophy phenotypes in human patients, see Baumbach et al.(1989) and Kunkel and Hoffman (1989). A comprehensive presentation of dystrophinopathies is found in Engel et al.(1994). In recent years various attempts have been made to restore in dystrophin-deficient fibres the expression of dystrophin (either complete or truncated) and of utrophin, a dystrophin-related protein. The final criterion in judging the success of gene therapy in Duchenne dystrophy is the correction of the pathological consequences of the absence of dystrophin. Hence, it is of