Pharmaceuticals used for the diagnosis, prevention, and treatment of rare diseases are known as orphan drugs. Rare diseases differ from common diseases in that they have a low prevalence rate and so appeal to a small patient population. These medications are being developed to treat people with extremely rare disorders for which there is now no therapy option. Pharmaceutical inventors are often discouraged from developing treatments for extremely limited patient groups due to the increasing expense of medication research in tandem with rigorous regulations, as well as the low return on investment. Orphan diseases are rare diseases that affect small patient groups and are consequently "orphaned" by the pharmaceutical industry, with only a few authorized pharmacological therapy choices available. Incentives for orphan drugs have sparked research into diseases with a high unmet medical need.