Scientists with two U.S. universities, the University of Washington and the University of Florida, used gene therapy to cure colorblindness in two squirrel monkeys.
The scientists said in a statement that the experiment was part of ongoing research into using gene therapy to treat adult vision disorders involving cone cells. Cone cells are the most important cells for vision in humans.
News agencies also report, Jay Neitz of the University of Washington in Seattle and his colleagues injected gene-carrying viruses into the retinas of two male squirrel monkeys, which are naturally colorblind. The gene carried instructions for the production of a protein known as opsin, which makes pigments that are sensitive to the colors red and green.
About five weeks after the treatment, the monkeys -- named Dalton and Sam -- began to develop the ability to see those colors, according to the results of detailed testing reported this week in the journal Nature.
"We knew right away when it began to work. It was as if they woke up and saw these colors. The treated animals unquestionably responded to colors that had been invisible to them," Neitz said in a statement released with the findings.
After more than 18 months of testing, the researchers were able to show that the animals could discern 16 hues, with some of the hues varying as much as 11-fold intensity, the researchers say. The animals continue to see color after more than two years.
In the meantime, the most common genetic disorder in humans, color-blindness affects about 3.5 million people in the United States, more than 13 million in China, and about 16 million in India, the study authors say.
Most color-blind people are men, and most function fine.