For the first time, genetically manipulated embryonic stem cells were transplanted into mice afflicted with muscular dystrophy(筋委縮症), and the rodents'(げっし類)muscle function improved. Muscular dystrophy is an umbrella term that covers nearly a dozen genetic diseases that primarily affect the muscles. The most common form of the disease, Duchenne muscular dystrophy (DMD), generally appears around age 3 or 4, and victims rarely live past 30. In tests measuring the muscle development of mice afflicted with DMD, the muscles of treated mice contracted with 3 times as much force as the muscles of mice that not been treated with the stem cells.