The respiratory deficiency of yeast strains previously assigned to complementation group G7 has been ascribed to the absence in the mutants of functional cytochrome b. Since G7 mutants are capable of synthesizing the apoprotein, the primary effect of the mutations is to prevent maturation of this electron carrier. The recombinant plasmid pG7/T1 with a 6.7-kilobase pairs (kb) insert of wild type yeast nuclear DNA has been selected from a genomic library by transformation of a G7 mutant to respiratory competency. The genetically active region of the pG7/T1 insert has been subcloned on a 3-kb fragment of DNA which has been shown to contain an open reading frame encoding a protein of 50,236 Mr. In situ disruption of the reading frame causes a deficiency in cytochrome b. The strain with the disrupted gene fails to complement G7 mutants thereby confirming the correct identification of the gene henceforth referred to as COR1. The carboxyl-terminal half of the COR1 gene has been fused to the amino-terminal half of the Escherichia coli trpE gene in the high expression vector pATH2. This plasmid construct promotes a high level of expression of the trpE/COR1 hybrid protein. Antibodies against the purified hybrid protein react with a 44 kDa protein subunit of yeast coenzyme QH2-cytochrome c reductase corresponding to the largest core subunit of the complex. These data indicate that the yeast nuclear gene COR1 codes for the 44-kDa core protein and that the latter is required for the conversion of apocytochrome b to mature cytochrome b.