We firstly addressed this by following a ‘panel within a panel’ approach, initially interrogating genes in which mutations were likely to result in the observed phenotypes (e.g. MUT, MCEE, ACSF3, ALDH6A1, MMAA, MMAB, SUCLA2, LMBRD1, ABCD4, MMADHC and MMACHC in patients with methylmalonic aciduria) and expanding our search when no likely pathogenic variants were identified. The gene discussed is MMAB; the disease is Methylmalonic aciduria.