A child in Europe has become the second individual ever to receive a commercial gene therapy, according to GlaxoSmithKline.

The treatment, called Strimvelis, can provide an outright cure for a rare inherited immune deficiency by revising a patient's genetic makeup (see “Gene Therapy’s First Out-and-Out Cure Is Here”).

Gene therapy has been widely explored in experimental medical studies, but its commercial potential is largely untested. Prior to now, only a single other individual, also from Europe, had ever accessed gene therapy to treat an inherited ailment outside of a clinical trial. That individual received a different drug, Glybera, in 2015.

On Tuesday, GlaxoSmithKline spokesperson Anna Padula said the company treated its first patient in March, nearly a year after Strimvelis was approved for sale in Europe in May 2016. The company declined to provide the nationality of the patient or say how the treatment was paid for.

In March, GlaxoSmithKline’s project leader for Strimvelis, Jonathan Appleby, said difficulty organizing cross-border European reimbursement for Strimvelis, which is offered at a single center in Milan, Italy, was to blame for the delay in commercialization.

The commercial fate of Strimvelis is being closely watched as several new gene-therapy treatments edge toward the market, including in the U.S., where Spark Therapeutics plans to seek approval for a blindness treatment this year.

Gene therapies are more complex than ordinary pills, and the diseases they treat affect few people. But sky-high prices have raised questions about whether patients can access them. Strimvelis, used to treat an ultra-rare immune deficiency, has a list price of 594,000 euros, or $648,000, making it one of the most expensive drugs available (see “Gene-Therapy Cure Has Money-Back Guarantee”).

Gene therapy has had a rocky start in the market. Last month, the biotech company UniQure said it would pull its gene therapy Glybera from the market in Europe after treating just a single patient. The company, which got a regulatory green light in Europe in 2012, blamed lack of demand. Its drug cost $1 million and had become known as the world’s most expensive medicine.