A complementation test helps in determining the presence of mutants in a gene or a functional unit. The test distinguishes the wild-type and the mutant phenotypes. Two recessive mutations on the opposite chromosomes indicate a mutant phenotype. Hence, it results in no complementation. Two recessive mutations present in the same chromosome indicate a wild-type phenotype. Hence, complementation occurs. Two mutants present on the same segment arise due to positive complementation. A diploid or a heterokaryon contributes to studying complementation of genes. Gene cloning by complementation of mutations includes a classic example of yeast genes. It includes an example of yeast-E. coli shuttle vector. These vectors replicate autonomously and in the yeast and E. coli cells. The yeast-E. coli shuttle vectors consist of beta-lactamase gene, an ampicillin resistance gene, pUC 18 sequence, and multiple cloning sites. Loss of beta-galactosidase function indicates appropriate DNA insertion in the multiple cloning sites. A genomic library consists of DNA fragments obtained from wild-type yeast. A genomic library transforms a mutant host yeast strain. It consists of two mutations. One mutant helps in selecting the transformants. It is known as ura3. Another mutation helps in searching the wild-type gene mutation. The wild-type gene required for the biosynthesis of arginine is known as ARG1 gene. The mutation in the gene known as arg1 mutation helps in the complementation test. Arginine amino acid plays an important role in yeast cell growth. Yeast cells lacking arginine or enzyme responsible for arginine biosynthesis do not grow well.