The simplest way to understand the function of a gene is to observe what happens in its absence. Following that line of inquiry, geneticists rely on patients who display certain symptoms and inquire about the loss or deficiency of which gene(s) may be responsible for these symptoms. Molecular biologists may instead choose a more direct line of approach and mutate or delete all or part(s) of a gene in a model system (which could be bacteria, cells, animals, or plants) and observe what happens to the physiology or behavior of the organism. Mutagenesis could also be employed in order to study what the role of specific amino acids or regions in the proteins are, or alternatively, to improve or change the characteristics of the protein encoded by the gene, for research purposes as well as commercial purposes such as drug development.