X-linked SMA is rare and mainly contains two forms (4): X-linked spinal and bulbar muscular atrophy (SMAX1, Kennedy disease, OMIM 313200: Online Mendelian Inheritance in Man 313200) and X-linked infantile spinal muscular atrophy [XL-SMA or SMAX2 (MIM301830)]. The gene discussed is UBA1; the disease is spinal muscular atrophy.