SDHB and inborn mitochondrial metabolism disorder: Although SDH was initially believed to have distinct genotype–phenotype relationships (SDHA and SDHAF1 being linked to mitochondrial disease, and SDHB/SDHC/SDHD/SDHAF2 being linked with cancer susceptibility), it is emerging that there is phenotypic overlap, prompting tumour surveillance of unaffected relatives heterozygous for SDHx mutations 45, 46.