Duchenne muscular dystrophy (DMD) is a severe type of muscular dystrophy that primarily affects boys. Muscle weakness usually begins around the age of four, and worsens quickly. Muscle loss typically occurs first in the thighs and pelvis followed by the arms. This can result in trouble standing up. Most are unable to walk by the age of 12. Affected muscles may look larger due to increased fat content. Scoliosis is also common. Some may have intellectual disability. Females with a single copy of the defective gene may show mild symptoms.The disorder is X-linked recessive. About two thirds of cases are inherited from a person's mother, while one third of cases are due to a new mutation. It is caused by a mutation in the gene for the protein dystrophin. Dystrophin is important to maintain the muscle fiber's cell membrane. Genetic testing can often make the diagnosis at birth. Those affected also have a high level of creatine kinase in their blood.Although there is no known cure, physical therapy, braces, and corrective surgery may help with some symptoms. Assisted ventilation may be required in those with weakness of breathing muscles. Medications used include steroids to slow muscle degeneration, anticonvulsants to control seizures and some muscle activity, and immunosuppressants to delay damage to dying muscle cells. Gene therapy, as a treatment, is in the early stages of study in humans. A small initial study using gene therapy has given some children improved muscle strength, but long term effects are unknown as of 2020.Various figures of the occurrence of DMD are reported. One source reports that it affects about one in 3,500 to 6,000 males at birth. Another source reports DMD being a rare disease and having an occurrence of 7.1 per 100,000 male births. A number of sources referenced in this article indicate an occurrence of 6 per 100,000.It is the most common type of muscular dystrophy. The median life expectancy is 28–30; however, with excellent care, some may live up to their 30s or 40s. The disease is much more rare in girls, occurring approximately once in 50,000,000 live female births.