IBA57 and Leber hereditary optic neuropathy: Based on the clinical phenotype, human MMDS3 patients may be classified into three categories (Liu et al., 2018): first is the most severe congenital type, leading to perinatal death and only observed in the two siblings; second is the most prevalent infantile type, with an age at onset of at least 4 months and variable outcome; and third is the comparably least affected childhood-type, with an age at onset of at least 3 years, slowly progressing spastic paraplegia, optic atrophy, and peripheral polyneuropathy, summarized as the SPOAN-like phenotype (Lossos et al., 2015).