FMR1 and proximal spinal muscular atrophy: For example, abnormal expansions in FMRP cause fragile X mental retardation syndrome (FXS), mutations in Survival motor neuron 1 (SMN1) are linked to spinal muscular atrophy (SMA), while mutations in TDP-43, FUS, optineurin (OPTN), and angiogenin (ANG) cause motor neuron diseases, including ALS (ref. 18).