Facioscapulohumeral muscular dystrophy (FSHD) is a genetic disorder that primarily affects the muscles of the face, shoulders, and upper arms. It is characterized by progressive muscle weakness and wasting, which can lead to difficulties in facial expressions, lifting the arms, and performing daily activities. Symptoms often begin in adolescence or early adulthood but can vary widely in severity and onset. FSHD is caused by a mutation in the DUX4 gene, leading to the abnormal expression of this gene in muscle cells. The condition is inherited in an autosomal dominant pattern, meaning that only one copy of the mutated gene from an affected parent can cause the disorder in their children.