Iso-1-cytochrome c and iso-2-cytochrome c are two of the few proteins of known primary structure from a microorganism which is particularly suitable for experimental genetic studies. The genetic and biochemical techniques for the iso-cytochrome c system have developed to a high degree of sophistication. The isolation of appropriate mutants have been facilitated by enrichment procedures for both forward and reverse mutations which are based, respectively, on defectiveness and functioning of cytochrome c for lactate utilization. A series of deletions are available, making it possible to conveniently map point mutants and to estimate their positions relative to the iso-1-cytochrome c sequence. The large number of mutants that have been characterized and the selection procuedures permit an unprecedented degree of genetic manipulation of nucleotide sequences by recombination. The sequencing of a portion of the gene from frameshift mutations has made it feasible to directly analyze the mRNA of iso-1-cytochrome c with a synthetic oligonucleotide. Thus the body of information concerning the iso-1-cytochrome c gene, the large number of defined mutants and the available genetic and biochemical techniques are without parallel for any other eukaryotic gene. This iso-cytochromes c system is being employed for investigating numerous problems in molecular biology and genetics, including: the isolation and characterization of structural and regulatory mutants for both iso-1 and iso-2-cytochrome c; the identification of amino acids inserted by various nonsense suppressors and other translational suppressors and the determination of the efficiencies of suppression; the determination of the specific action of numerous mutagens; gene conversion studies and the examination of the relationship of recombination frequencies and nucleotide alterations; the use of E. coli plasmids for cloning the CYC1 and CYC7 genes which determine, respectively, iso-1 and iso-2-cytochrome c; the characterization of mRNAs specifying the two iso-cytochromes c and their measurements in normal and mutant strains.