PRPS1 and Leber hereditary optic neuropathy: Mildly compromised PRPS1 activity caused by loss-of-function mutations leads to X-linked non-syndromic sensorineural hearing loss (DFNX-2) [13], and severe loss of PRPS1 activity in a missense mutation is often represented by the more severe Arts syndrome, in which patients present with a “triad” of preverbal hearing loss, early-onset optic atrophy, and severe sensory neuromotor neuropathy [14].