EXOSC3 and proximal spinal muscular atrophy: Recessive variants in EXOSC3 (MIM: 606489) and EXOSC8 (MIM: 606019), encoding EXOSC3 and EXOSC8, respectively, of the human exosome, cause pontocerebellar hypoplasia type 1 (PCH1) of variable severity with spinal muscular atrophy (SMA) and hypomyelination of the central nervous system (CNS).16, 17EXOSC3 variants account for about 40% of PCH1 cases worldwide,18 suggesting further genetic heterogeneity.