Mutations p.M115V and p.V309F are located in the trimeric interface of PRPS1 and destabilize the ATP-binding site and the allosteric site I. The mutation p.A121G (c.362C > G), which has been associated with CMTX5 and early-onset hearing loss but not with optic atrophy, may reflect a loosening of the interaction between the hexamers or a minimally ordered catalytic site [177]. The gene discussed is PRPS1; the disease is hereditary optic atrophy.