respiratory muscles, and other areas. Fatigue is common. Signs usually appear before age five, and may even be observed from the moment a boy takes his first steps. There is general difficulty with motor skills, which can result in an awkward manner of walking, stepping, or running. They tend to walk on their toes, in part due to shortening of the Achilles tendon, and because it compensates for knee extensor weakness. Falls can be frequent. It becomes harder and harder for the boy to walk; his ability to walk usually completely disintegrates before age 13. Most men affected with DMD become essentially "paralyzed from the neck down" by the age of 21. Cardiomyopathy, particularly dilated cardiomyopathy, is common, seen in half of 18-year-olds. The development of congestive heart failure or arrhythmia (irregular heartbeat) is only occasional. In late stages of the disease, respiratory impairment and swallowing impairment can occur, which can result in pneumonia. A classic sign of DMD is trouble getting up from lying or sitting position, as manifested by a positive Gowers's sign. When a child tries to arise from lying on his stomach, he compensates for pelvic muscle weakness through use of the upper extremities: first by rising to stand on his arms and knees, and then "walking" his hands up his legs to stand upright. Another characteristic sign of DMD is pseudohypertrophy (enlarging) of the muscles of the tongue, calves, buttocks, and shoulders (around age 4 or 5). The muscle tissue is eventually replaced by fat and connective tissue, hence the term pseudohypertrophy. Muscle fiber deformities and muscle contractures of Achilles tendon and hamstrings can occur, which impair functionality because the muscle fibers shorten and fibrose in connective tissue. Skeletal deformities can occur, such as lumbar hyperlordosis, scoliosis, anterior pelvic tilt, and chest deformities. Lumbar hyperlordosis is thought to be compensatory mechanism in response to gluteal and quadriceps muscle weakness, all of which cause altered posture and gait (e.g.: restricted hip extension). Non musculoskeletal manifestations of DMD occur. There is a higher risk of neurobehavioral disorders (e.g., ADHD), learning disorders (dyslexia), and non-progressive weaknesses in specific cognitive skills (in particular short-term verbal memory), which are believed to be the result of absent or dysfunctional dystrophin in the brain. Cause DMD is caused by a mutation of the dystrophin gene, located on the short arm of the X chromosome (locus Xp21) that codes for dystrophin protein. Mutations