SDHB and hyperinsulinemic hypoglycemia, familial, 4: Of the four known SDH assembly factors, pathogenic variants resulting in mitochondrial complex II deficiency have only been reported in SDHAF1. The absence of pathogenic variants in SDHAF2-4 might suggest that there are other proteins capable of compensating for defective SDHAF2, SDHAF3 or SDHAF4 but the task of incorporating Fe-S clusters into SDHB lies solely with SDHAF1 and this unique function makes it indispensable.