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Now, some good news for the tens of millions of Americans who are colorblind. Scientists have used gene therapy to achieve full color vision in two monkeys that were born unable to tell red from green. NPR's Jon Hamilton has more.

JON HAMILTON: The monkeys are male squirrel monkeys named Sam and Dalton. Jay Neitz, a researcher at the University of Washington, says female squirrel monkeys can see the full range of colors, but males, like Sam and Dalton, can't.

Professor JAY NEITZ (Ophthalmology, University of Washington): For these monkeys, there's only two colors, blue and yellow. And if you mix blue and yellow to them, you just get a gray color.

HAMILTON: That's because male squirrel monkeys are born without a receptor in their eyes that would let them see red and green. Neitz was part of a team that thought if they could get these monkeys to see the full spectrum, it might make it possible to do the same thing in people who are colorblind.

The team developed a gene therapy technique that would reprogram some of the color receptors in the monkeys' eyes. But Neitz knew that even if the therapy worked, there was no guarantee the monkeys' new eyes would be able to communicate with their old brains.

Prof. NEITZ: I went out and asked all my neuroscientist friends: If I do this in an adult monkey, will this give them color vision? And everyone said, absolutely not.

Because just think about it, that you don't have any of the wires to carry the information. You don't have any of the circuitry to interpret it properly.

HAMILTON: Neitz and his team tried the experiment anyway. Dalton, named after the British scientist who originally described colorblindness, got treated first. For several months, there was no change. Every day, Dalton tried to win a juice reward by touching a red or green target on a computer screen. And every day, he failed.

Neitz was anxious but not surprised. Previous experiments had shown that it took about five months to reprogram a critical mass of receptor cells. Then, just about five months after his gene therapy, Dalton started hitting the target every time.

Prof. NEITZ: At first we told ourselves, you know, maybe he's just guessing really well today or something. But over a few days, it became very clear that he wasn't just guessing. That floored us.

HAMILTON: Later on, the other squirrel monkey, Sam, would have a similar epiphany. Neitz says both monkeys have seemed pretty pleased with themselves ever since.

Prof. NEITZ: It's just like a kid that goes in, and they don't want to get wrong answers on a test. The monkeys kind of act that way. And they're very happy to be straight-A students now.

HAMILTON: Scientists who study color vision are calling the story of Sam and Dalton dazzling and surprising. Gerry Jacobs, from the University of California, Santa Barbara, says it's remarkable that the technique worked at all on an adult animal.

Earlier research had shown that after childhood, the brain loses much of its ability to adapt to new types of visual information. But Jacobs says color seems to be different. The adult brains of even colorblind monkeys and humans appear ready to receive new color input.

Professor GERALD JACOBS (University of California, Santa Barbara): The primate is really almost an ideal place to have this happen, because they have a pathway through the retina, which in most primates is almost designed to extract color information.

HAMILTON: Robert Shapley, of New York University, says what amazed him was how quickly the monkeys' brains responded once the reprogrammed cells began working.

Professor ROBERT SHAPLEY (New York University): As soon as that happened, the animals were able to start doing color discrimination. I mean, it was almost no delay. That was really striking.

HAMILTON: Shapley says that bodes well for people with colorblindness.

Prof. SHAPLEY: There's no reason to think that these monkeys and humans are dramatically different in the way that they are interpreting color signals from the eye.

HAMILTON: And researchers say gene therapy might eventually offer a way to treat a range of vision problems. The new research appears in the journal Nature.

Jon Hamilton, NPR News.

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