Drosophila is a promising tool to study the pathophysiology of BCS1L-related mitochondrial complex III deficiency, and to test novel treatment strategies, although the puzzling results of the GRACILE mutation in young mice plus the consistent absence of homozygous loss of function mutations of BCS1 in humans and the severity of partial loss of Bcs1 in flies should prompt to hypothesize an additional, yet unknown role of BCS1 that can account for its dispensability in CIII biogenesis in young mice, in contrast with its indispensability in humans and flies. This evidence concerns the gene BCS1L and hyperinsulinemic hypoglycemia, familial, 4.