is a gradually progressive neuromuscular disorder chiefly characterized by wasting of the proximal muscles (those closer to the trunk) and bulbar muscles (those of the face and throat). The condition mainly affects males, with onset between the ages of 30 and 60. Early signs and symptoms may include tremor, muscle cramps and muscle twitching, followed by progressive muscle weakness and wasting which may manifest in a variety of ways. Affected individuals may also have gynecomastia, testicular atrophy (reduction in size or function), and reduced fertility. It is caused by a mutation in the androgen receptor (AR) gene, in which a DNA segment known as a CAG triplet repeat is abnormally expanded. It is inherited in an X-linked recessive manner. Treatment may include physiotherapy and rehabilitation, medications for tremor and muscle cramps, and hormone therapy or surgical treatment of gynecomastia. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.