The frameshift variant c.845_846del in the Cytochrome C oxidase assembly Factor SURF1 (rs782316919) is associated with Leigh syndrome due to mitochondrial complex IV deficiency (VCV000012770.12) and has been reported earlier as the most prevalent SURF1 disease allele among Russian, Ukrainian, and Polish patients (Piekutowska-Abramczuk et al., 2009; Tsygankova et al., 2010). The gene discussed is SURF1; the disease is hyperinsulinemic hypoglycemia, familial, 4.