To ensure that all patients reported with confirmed mitochondrial complex II disease consequence to SDHx defects were collated, the following key terms were used to search on PubMed (https://www.ncbi.nlm.nih.gov/pubmed): “mitochondria”, “complex II deficiency”, “SDHA”, “SDHB”, “SDHC”, “SDHD”, “SDHAF1”, “SDHAF2”, “SDHAF3”, “SDHAF4”, “pathogenic variants”, “mutations”. Here, SDHAF1 is linked to hyperinsulinemic hypoglycemia, familial, 4.