LONDON (AP) ― A small, preliminary study using gene therapy to treat a rare form of blindness is promising and could trigger similar efforts for other causes of vision loss, British doctors say.

They studied just six patients. Of those, two have had dramatic improvements in their vision and none has reported any serious side effects. The study was only designed to test the treatment’s safety, not its effectiveness.

Gene therapy ― inserting copies of a normal gene into a patient who has a faulty or missing gene ― has previously been tried for other rare types of blindness with limited success.

All of the patients in the new trial were men with choroidermia, an inherited and untreatable form of progressive blindness. The disease is the result of a protein deficiency that kills eye cells that detect light.

“We’re trying to rescue cells that would otherwise have died,” said Dr. Robert MacLaren of the University of Oxford. He and colleagues injected a harmless virus carrying the missing protein into the thinnest part of the patients’ retinas. The study was paid for by the Wellcome Trust and Britain’s Department of Health, and was published online Thursday in the journal, Lancet. Dr. Ian MacDonald, chairman of ophthalmology and visual sciences at the University of Alberta in Canada, who was not part of the study but is planning a similar trial, called the new work “very promising because there is really no other way to deliver this protein.”