High Quality Content by WIKIPEDIA articles! X-linked spinal muscular atrophy 2 (SMAX2) is a form of spinal muscular atrophy. IT is similar to but distinguishable from Werdnig-Hoffmann disease. TYpically it manifests at or before birth, which is unusual for SMN-SMA, and it only affects boys, where SMN-SMA affects both sexes equally. THe "2" is used to distinguish the condition from Kennedy disease. HOwever, the "2" is not always used. IN this context, the condition may simply be abbreviated "XL-SMA". IT is associated with UBE1. SPinal Muscular Atrophy (SMA) is a neuromuscular disease characterized by degeneration of motor neurons, resulting in progressive muscular atrophy (wasting away) and weakness.