Until recently, medicine has had little to offer most of the millions of patients suffering from rare and ultrarare genetic conditions. But the development in 2019 of Milasen, the first genetic intervention developed for and administered to a single patient suffering from an ultrarare genetic disorder, has offered hope to patients and families. In addition, Milasen raised a series of conceptual and ethical questions about how individualised genetic interventions should be developed, assessed for safety and efficacy and financially supported. The answers to these questions depend in large part on whether individualised therapies are understood as human subjects research or clinical innovation, different domains of biomedicine that are regulated by different modes of oversight, funding and professional norms. In this article, with development and administration of the drug Milasen as our case study, we argue that at least some individualised genetic therapies are not, as some have argued, either research or treatment. Instead, they are research-treatment hybrids, a category that has both epistemological and pragmatic repercussions for funding, ethics oversight and regulation.