WASHINGTON (AP) — U.S. health officials approved the nation’s first gene therapy for an inherited disease, a treatment that improves the sight of patients with a rare form of blindness. It marks another major advance for the emerging field of genetic medicine.

The approval for Spark Therapeutics offers a life-changing intervention for a small group of patients with a vision-destroying genetic mutation and hope for many more people with other inherited diseases. The drugmaker said it will not disclose the price until next month, delaying debate about the affordability of a treatment that analysts predict will be priced around $1 million.

The injection, called Luxturna, is the first gene therapy approved by the Food and Drug Administration for an inherited disease and the first in which a corrective gene is given directly to patients. The gene mutation interferes with the production of an enzyme needed for normal vision.

Patients who got the treatment have described seeing snow, stars or the moon for the first time.

“One of the best things I’ve ever seen since surgery are the stars. I never knew that they were little dots that twinkled,” said Mistie Lovelace of Kentucky, one of several patients who urged the FDA to approve the therapy at a public hearing in October.

Patients with the condition generally start losing their sight before 18, almost always progressing to total blindness. The defective gene that causes the disease can be passed down for generations undetected before suddenly appearing when a child inherits two copies from their parents. Between 1,000 and 2,000 people in the U.S. have the condition.

Luxturna is delivered via two injections — one for each eye — that replaces the defective gene that prevents the retina, tissue at the back of the eye, from converting light into electronic signals sent to the brain.

The FDA has approved three gene therapies since August, as decades of research into the genetic building blocks of life begin translating into marketable treatments. The previous two are custom-made treatments for forms of blood cancer. Novartis’ Kymriah is priced at $475,000 for a one-time infusion of genetically enhanced cells. Gilead Sciences’ similar treatment, Yescarta, costs $373,000 per treatment.

Spark Therapeutics said it will announce its drug’s price in early January, but suggested its own analysis put the value of the therapy in the $1-million-dollar range. Key to the company’s reasoning is the assumption that Luxturna will be given once, with lasting benefits. To date, the company has tracked patients enrolled in a key study for as long as four years and hasn’t seen their vision deteriorate.