The treatment for XLH that's currently in clinical trials, KRN23, was recently granted "orphan drug status" in the European Union.
As explained in the press release from Ultragenyx, "The European Commission grants orphan drug status for medicinal products intended to treat diseases or conditions that affect fewer than five in 10,000 people in the European Union. The designation provides certain benefits and incentives in the EU, including protocol assistance, fee reductions, and ten years of market exclusivity once the medicine is on the market."
Getting this designation is an important step toward bringing KRN23 to the market in the EU. Orphan drug status was already granted in the US in late 2009.
You can read the press release here: