Fig. 1 NDUFV1 mutations in patients with isolated complex I deficiency. a, b, Two patients were compound heterozygotes for the 175C→ T (R59X) and the 1268C→ T (T423M) transitions. Due to nonsensemediated messenger decay of the mRNA with the premature stop codon, the 1268C→ T transition appears to be homozygous on cDNA in both affected individuals. c, The third patient is homozygous for the 1022C→ T transition, which changes a conserved amino acid sequence from alanine to valine. The codons are indicated above and the reading frame below the electropherograms. d− i, Segregation of NDUFV1 mutations in families A and B. d, Pedigree of family A: the black half-filled symbols indicate the R59X mutation and the gray halffilled symbols the T423M mutation. Both affected children are compound heterozygotes. The results of the restriction analysis are depicted below the symbol of the respective family member. e, Restriction analysis of the 175C→ T (R59X) transition on genomic DNA, which abolishes a TaqI site. The father and both patients are heterozygous for this mutation. f, Restriction analysis of the 1268C→ T transition (T423M) on genomic DNA, which creates a new NdeI site. The mother and both patients are heterozygous for this mutation. g, Quantitative radioactive restriction analysis of the 1268C→ T transition on cDNA. The mother is heterozygous for this mutation. The children, however, show only a faint signal where the wild-type band is expected (open arrows). This indicates that the mRNA (from the father’s allele) with a premature stop codon is subjected to nonsense-mediated messenger decay. h, Pedigree of family B. i, Restriction analysis of the 1022C→ T (A341V) transition on genomic DNA, which abolishes an HhaI site. The patient (B. II: 1) is homozygous and all other family members are heterozygous for this mutation; a normal control is shown on the far right. A standard 100-bp DNA reference ladder is shown on the right side of each electropherogram. wt, wild type; mut, mutant. a b c d h i e f g correspondence

260 nature genetics• volume 21• march 1999