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. This evidence concerns the gene SDHB and mitochondrial disease.