Using exogenous genes to reduce the set of essential amino acids?
Decreasing the number of essential amino acids is frequently used as a selection marker during molecular biology experiments: An organism (usually yeast) that cannot synthesize a certain amino acid is first grown in the presence of said amino acid in the media. Then it is transfected with a gene of interest together with the gene that enables it to synthesize the amino acid. Now being grown in a culture medium that does not contain the amino acid, only individuals that were successfully transfected can grow, the others will starve.

A similar approach was also used in  Beadle and Tatum's Nobel prize-winning work on the one gene-one enzyme hypothesis.