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Strimvelis is the first ex-vivo stem cell gene therapy to treat patients with a very rare disease called ADA-SCID (Severe Combined Immunodeficiency due to Adenosine Deaminase deficiency). ADA-SCID is estimated to occur in approximately 15 patients per year in Europe.

The treatment is personalized for each patient; hematopoietic stem cell (HSCs) are extracted from the patient and purified so that only CD34-expressing cells remain. Those cells are cultured with cytokines and growth factors and then transduced with a gammaretrovirus containing the human adenosine deaminase gene and then reinfused into the patient. These cells take root in the person's bone marrow, replicating and creating cells that mature and create normally functioning adenosine deaminase protein, resolving the problem.

The treatment was developed at San Raffaele Telethon Institute for Gene Therapy and developed by GlaxoSmithKline (GSK) through a collaboration with Fondazione Telethon and Ospedale San Raffaele. GSK, working with the biotechnology company MolMed S.p.A, developed a manufacturing process. In April 2016, a committee at the European Medicines Agency (EMA) recommended marketing approval for its use in children with adenosine deaminase deficiency, for whom no matched hematopoietic stem cell donor is available, on the basis of a clinical trial that produced a 100% survival rate; the median follow-up time was 7 years after the treatment was administered. Around 75% of people who received the treatment needed no further enzyme replacement therapy. Strimvelis was approved by the European Commission on 27 May 2016. As of 2016, the only site approved to manufacture the treatment was MolMed in Italy. In 2017, GSK announced it was looking to sell off Strimvelis, and in March 2018 GSK sold Strimvelis to Orchard Therapeutics Ltd.