BCS1L encodes a mitochondrial AAA+-family chaperone essential for complex III biogenesis, specifically mediating insertion of the Rieske iron-sulfur protein (RISP/UQCRFS1) into the respiratory chain 1. This function is critical for maintaining mitochondrial respiratory chain assembly and tubular network organization. BCS1L mutations represent the most frequent cause of isolated complex III deficiency, causing a spectrum of mitochondrial diseases termed BCS1L Mitopathies 2. Clinical presentations range from mild Björnstad syndrome (characterized by pili torti and sensorineural hearing loss) 3 to severe GRACILE syndrome (growth restriction, aminoaciduria, cholestasis, iron overload, and lactic acidosis with early mortality) 4. Intermediate complex III deficiency presents with multisystem involvement including hepatic dysfunction, neurological impairment, and developmental delays 5. Pathogenic mechanisms involve impaired complex III assembly and diminished mitochondrial respiration, with emerging evidence suggesting altered BCS1L-CIII interactions preventing proper RISP loading 1. Disease severity correlates with mutation location within BCS1L's structural domains 2. The wide phenotypic heterogeneity reflects variable impacts on respiratory chain function and tissue-specific mitochondrial demands, with liver and brain most frequently affected 6.