In a finding hailed as an exciting step toward treating humans for the most common form of muscular dystrophy, scientists have corrected a muscle defect in mice by injecting immature muscle cells.

Mice that could not make a protein called dystrophin in their muscles began to produce it after the injections, two teams of researchers report.In humans, lack of the protein causes Duchenne muscular dystrophy, the most common and severe form of the disease.

The research represents "the most exciting approach for human therapy that, in my opinion, has ever come along," said Donald Wood, director of research for the Muscular Dystrophy Association.

Preliminary experiments in a limited number of Duchenne patients may begin this summer, after researchers from several nations meet in June to discuss the issues involved, Wood said.

Those studies, focusing on single muscles or a few muscles, could lead to larger human studies. It is too early to say when the injection procedure could be made widely available, if it proves useful, Wood said.

"We basically have the first step toward a potential to do therapeutics. It looks promising," said researcher Louis Kunkel of Harvard Medical School in a recent interview.

Their study is "extremely exciting work," said Theodore Munsat, professor of neurology and pharmacology at the Tufts University Medical School and director of neuromuscular research at Boston's New England Medical Center Hospital.

Duchenne muscular dystrophy is a genetic disorder that strikes boys almost exclusively, appearing in about one in every 3,500 male births in the United States. It causes a progressive weakening and wasting of voluntary muscles. Most patients must use wheelchairs by age 12, and most die in their early 20s.

The experiments used immature muscle cells called myoblasts, which normally help repair muscle fibers that have broken because of injury or strenuous exercise.

Researchers injected myoblasts into mice that lacked dystrophin and found that the myoblasts fused with muscle fibers. Once inside a fiber, the myoblast nucleus provided a gene that allowed the fiber to begin producing dystrophin.