Duchenne muscular dystrophy is caused by a mutated gene on the X chromosome that prevents the production of dystrophin, a protein vital for muscle integrity. This leads to progressive muscle degeneration over time, initially impacting the muscles of the legs and pelvis but eventually weakening the respiratory muscles. As a result, patients with Duchenne muscular dystrophy experience a gradual decline in lung function and respiratory failure is a major cause of mortality if left untreated.