The c.64T>C, p.Trp22Arg NDUFB3 variant is represented on the ExAC server (0/81/121214 (homozygous/heterozygous/alleles); MAF=6.6×10−4) and has been reported in the literature twice previously, once in compound with a nonsense mutation and once as a homozygote; functional complementation experiments confirmed NDUFB3 as the causative gene defect in both cases.9, 10 The homozygous case reported by Calvo et al9 had IUGR (weight <3rd centile) and presented with hypotonia and lactic acidosis, required ventilation and died at 4 months of age. This evidence concerns the gene NDUFB3 and lactic acidosis.