An experimental stem-cell treatment has restored the sight of a man blinded by the degeneration of his retinal cells. The man, who is taking part in a trial examining the safety of using human embryonic stem cells (hESCs) to reverse two common causes of blindness, can now see well enough to be allowed to drive.

People undergoing treatment had reported modest improvements in vision earlier in the trial, which began in 2011, but this individual has made especially dramatic progress. The vision in his affected eye went from 20/400 essentially blind to 20/40, which is considered sighted.

"There's a guy walking around who was blind, but now can see," says Gary Rabin, chief executive officer of Advanced Cell Technology, the company in Marlborough, Massachusetts that devised the treatment. "With that sort of vision, you can have a driver's licence."

In all, the company has so far treated 22 patients who either have dry age-related macular degeneration, a common condition that leaves people with a black hole in the centre of their vision, or Stargardt's macular dystrophy, an inherited disease that leads to premature blindness. The company wouldn't tell New Scientist which of the two diseases the participant with the dramatic improvement has.

In both diseases, people gradually lose retinal pigment epithelial (RPE) cells. These are essential for vision as they recycle protein and lipid debris that accumulates on the retina, and supply nutrients and energy to photoreceptors the cells that capture light and transmit signals to the brain.

The company is testing treatments for both conditions by turning hESCs into fresh RPE cells, then giving each trial participant a transplant of the cells beneath the retina in one eye.

Although the aim of the trial is primarily to check that the stem cells are safe, participants have reported improvements in their sight. The company intends to publish the outcomes in full when all the results are in.

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Stem-cell treatment restores sight to blind man