Exclusive rights to sell known, unpatented, or off-patent compounds, for particular therapeutic purposes, typically for five to ten years. These rights were developed in both the United States and Europe to encourage the development of drug therapies from compounds without patent protection, particularly where the therapeutic market is small and therefore does not usually justify the cost of extensive trials. There is some controversy about the application of these rules in Europe, mainly about the appropriateness of the compounds that have received orphan drug status.