While their prevalence in the United States (US) may be relatively low, ‘rare’ diseases have been garnering increasing attention in recent years through such channels as product approvals, official guidance documents, and public meetings. Over the past few months, three new biologic treatments for rare diseases have entered the US market, an unusually high number. Deborah A. Komlos of Clarivate concludes that one of the key challenges with rare diseases is the likelihood of having relatively small study populations.