SBMA, also known as Kennedy's disease, is an inherited and rare neuromuscular disorder, caused by a polyglutamine-expanded sequence in the androgen receptor (AR) protein as a result of a CAG repeat expansion in the AR gene which is localized on the X chromosome. The symptoms occur in the age range of 18 and 64 years, but generally appear according to the length of the CAG repeat, in the mid-40s. Clinical findings include slow degeneration of muscles and motor neurons resulting in muscle weakness, atrophy, and fasciculation. Several mechanisms are involved in the molecular pathogenesis of SBMA, including disruption of the transcriptional regulation, protein homeostasis, intracellular trafficking, and cellular signaling. Although all other polyglutamine diseases show autosomal dominant inheritance, X-linked recessive inheritance is observed in SBMA, thus it mostly occurs in males. The majority of carrier women with the allele containing a pathogenic CAG expansion (CAG>37) are asymptomatic, with only a small minority having milder symptoms, such as cramps and tremor.