Becker muscular dystrophy (BMD) is an X-linked recessive disorder caused by a mutation in the dystrophin gene located on the X chromosome. The mutation leads to reduced dystrophin protein, which is important in muscle cell structural stability. Males are affected, although females can be carriers. The incidence is about 1 in 30 000 male infants.

BMD is similar to Duchenne muscular dystrophy (DMD) but typically has a later onset, around age 10 or older, and a milder course. Symptoms include progressive proximal muscle weakness and atrophy of the legs and pelvis, which often leads to difficulty walking or climbing stairs, myalgias with exertion, pseudohypertrophy of the calves, and fatigue. Cognition is often normal. Neck flexion strength is typically preserved, and contractures are not as common or severe as in DMD. However, cardiac disease (cardiomyopathy) is often more apparent and a more common cause of death than in DMD. Later in the course of disease, there can be arm weakness, scoliosis, dysphagia, and respiratory failure. Prognosis is variable, but patients typically remain ambulatory until age 16 or older and usually survive past age 30.