Additional Relevant Details The Orphan Drug Act provides for economic incentives to encourage the development of drugs for diseases affecting fewer than 200,000 people in the United States. Orphan drug designation entitles seven years of market exclusivity in the United States if market approval is granted for IONIS-HTTRx for the treatment of patients with HD. Additional incentives include tax credits related to clinical trial expenses, an exemption from the FDA-user fee, and FDA assistance in clinical trial design.