Mutations in SCO2, a protein required for the proper assembly and functioning of cytochrome c oxidase (COX; complex IV of the mitochondrial respiratory chain), cause a fatal infantile cardioencephalomyopathy with COX deficiency. We have generated mice harboring a Sco2 knock-out (KO) allele and a Sco2 knock-in (KI) allele expressing an E-->K mutation at position 129 (E129K), corresponding to the E140K mutation found in almost all human SCO2-mutated patients. Whereas homozygous KO mice were embryonic lethals, homozygous KI and compound heterozygous KI/KO mice were viable, but had muscle weakness; biochemically, they had respiratory chain deficiencies as well as complex IV assembly defects in multiple tissues. There was a concomitant reduction in mitochondrial copper content, but the total amount of copper in examined tissues was not reduced. These mouse models should be of use in further studies of Sco2 function, as well as in testing therapeutic approaches to treat the human disorder.