SDHB and inborn mitochondrial metabolism disorder: As with other mitochondrial diseases, there are no definitive genotype-phenotype correlations; the initial clear association between complex II deficient Leigh syndrome and bi-allelic pathogenic SDHA variants has been blurred by reports of SDHA-deficient cases presenting with cardiomyopathy [27,31], and conversely with complex II deficient Leigh syndrome cases subsequently found to harbour defects in other SDHx genes – SDHAF1 [40,42], SDHB [31] and SDHD [38].