Plasmids contribute significantly to bacterial genetic diversity and plasticity by encoding functions that might not be specified by the bacterial chromosomal DNA. Plasmids specify traits that allow the host to persist in environments that would otherwise be either lethal or restrictive for growth. For example antibiotic resistance and protein expression. Antibiotic resistance genes are often encoded by the plasmid, which allows the bacteria to persist in an antibiotic containing environment, thereby providing the bacterium with a competitive advantage over antibiotic-sensitive species. As a tool, plasmids can be modified to express the protein of interest (e.g., production of human insulin using recombinant DNA technology).