Charcot-Marie-Tooth disease-5 (CMTX5, MIM 311070 [1,2]), Arts syndrome (MIM 301835 [3,4]) and X-linked nonsyndromic sensorineural deafness (DFN2; MIM 304500 [5]) present three clinically distinct but genetically allelic disorders, caused by reduced phosphoribosylpyrophosphate synthetase 1 (PRS1) activity due to PRPS1 mutations [6]. The gene discussed is PRPS1; the disease is Lethal ataxia with deafness and optic atrophy.