Complementation of mutation- In this approach, a mutant host strain can be used to screen the plasmid containing the missing gene and the transformant will grow only if the gene product from clone will complement the function. In general genes taking part in metabolic pathway or biosynthetic pathway are routinely been used for this purpose.

There are 3 important requirement for this approach to work-

1. Host strain deficient in a particular gene. If the gene belongs to the biosynthetic pathway, the mutant host in this case are called as auxotroph as host dependent on the gene product or final product of the biosynthetic pathway as a supplement in media for growth.

2. A defined media with missing nutrient.

3. A vector containing the gene to supply the gene product to complement.