Mutations in HYLS1 underlie hydrolethalus syndrome (MIM: 236680), a perinatal lethal developmental disorder characterized by severe brain malformation including hydrocephalus and absent midline structures (Mee et al. 2005), as well as Joubert syndrome (MIM: 213300), a milder disorder characterized by defects in the cerebellum and brain stem leading to impaired balance and coordination (Oka et al. 2016), suggesting the HYLS1 P119L mutation may have had adverse developmental consequences. This evidence concerns the gene HYLS1 and Joubert syndrome.