The most severe neurodegenerative disorders, with encephalopathy and psychomotor retardation or regression, are nuclear types 1, 2, 4, 7, 8 and 9 of mitochondrial complex III deficiency, respectively associated with mutations of the BCS1L, TTC19, UQCRQ, UQCC2, UQCC3 and LYRM7 genes, respectively encoding mitochondrial chaperone BCS1, mitochondrial tetratricopeptide repeat protein 19 (both required for complex III assembly), cytochrome b-c1 subunit 8, ubiquinol-cytochrome-c reductase assembly factors 2 and 3, and LYRM7 (an assembly factor for Rieske Fe-S protein). This evidence concerns the gene BCS1L and hyperinsulinemic hypoglycemia, familial, 4.