mutations in nuclear genes associated with defective complex iii ( ciii ) of the mitochondrial respiratory chain are rare , having been found in only two ciii assembly factors and , as private changes in single families , three ciii structural subunits . 
 recently , human lyrm7/mzm1l , the ortholog of yeast mzm1 , has been identified as a new assembly factor for ciii . in a baby patient with early onset , 
 severe encephalopathy , lactic acidosis and profound , isolated ciii deficiency in skeletal muscle , we identified a disease - segregating homozygous mutation ( c.73g > a ) in lyrm7/mzm1l , predicting a drastic change in a highly conserved amino - acid residue ( p.asp25asn ) . in a mzm1 yeast strain , 
 the expression of a mzm1d25n mutant allele caused temperature - sensitive respiratory growth defect , decreased oxygen consumption , impaired maturation / stabilization of the rieske fe  s protein , and reduced complex iii activity and amount . 
 lyrm7/mzm1l is a novel disease gene , causing ciii - defective , early onset , severe mitochondrial encephalopathy .