There are a number of clinical and biochemical features of RMND1 mutations, for example, congenital sensorineural deafness, lactic acidaemia, hypotonia and multiple mitochondrial respiratory chain deficiencies, which are also described in other genetic causes of mitochondrial disease with renal involvement.12 While this is true, clinical features are emerging that are suggestive of a particular aetiology—our case series shows that RMND1 mutations are associated with both renal tubular acidosis type 4 (hyponatraemia and hyperkalaemia) and cystic/hypoplastic kidneys. The gene discussed is RMND1; the disease is Hyponatremia.