EXOSC3 and proximal spinal muscular atrophy: Wan et al. (17) showed for the first time that mutations in the exosome complex can cause human disease, when they reported mutations in the EXOSC3 sub-unit of the exosome in pontocerebellar hypoplasia and spinal muscular atrophy (SMA) (PCH1), accounting for about 40% of cases worldwide (18).