UBA1 and proximal spinal muscular atrophy: Mutations in the ubiquitin-activating enzyme 1 (UBA1; previously UBE1) have been reported to cause infantile-onset X-linked SMA (SMAX2; Ramser et al., 2008; Dlamini et al., 2013) and the levels of UBA1 were reduced by ~50% in SMA mouse spinal cord (Huang et al., 2009b), more than 60% in skeletal muscle (Huang et al., 2009a) and by 50% in SMA mouse Schwann cells (Aghamaleky Sarvestany et al., 2014).