Although Pfizer beat Sarepta to start a Phase 3 trial of its gene therapy, the smaller biotech is claiming a milestone in initiating the study in the U.S. as well as Europe and Asia. Pfizer’s trial, by contrast, is currently only enrolling patients in Europe and Asia, though the pharma plans to eventually do so in the U.S., too.

Sarepta may also hold an advantage in having previously studied its gene therapy, dubbed SRP-9001, against a placebo. Pfizer’s Phase 3 trial, which began in January, was the first placebo-controlled test of the treatment.

SRP-9001 is designed to replace the faulty gene that causes Duchenne, in theory enabling patients to produce a modified version of dystrophin, a key muscle-building protein that they lack due to their disorder.

But Sarepta has faced its own set of hurdles in getting to Monday’s milestone.

In January, the company’s therapy did not significantly improve motor function versus placebo in 4- to 7-year old boys enrolled in a Phase 2 study. After 48 weeks, testing showed the gene therapy led to higher dystrophin production, but the difference in motor function between the patients who received the therapy and those in the placebo group was not statistically significant.

The missed goal set Sarepta back in its goal of applying for FDA approval while a larger Phase 3 trial was running at the same time. CEO Doug Ingram had previously indicated the company could pair Phase 2 results with proof that a commercial-grade form of its treatment was as potent as the clinical-stage product, a path to approval that was closed off by the disappointing data.

In May, Sarepta disclosed that study results showed its commercial-grade gene therapy did perform as well as its clinical-grade product.

Pfizer, meanwhile, is facing a setback of its own, announcing in late September that several patients being treated with its gene therapy developed serious side effects involving muscle weakness. The pharma changed the design of its Phase 3 trial as a result.