Researchers at the University of Washington in Seattle have successfully used gene therapy to cure color blindness in adult monkeys. Red-green color blindness, which results from loss of either the red- or green-sensitive visual pigment in the eye, is the most common genetic disorder; affecting 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. The work is a culmination of more than 8 years of research performed in collaboration with laboratories at the University of Florida and the Medical College of Wisconsin. Color Blindness Cured

In the study published in the journal Nature, researchers presented squirrel monkeys, some colorblind, some not, with a touch screen filled with grey dots. In a test similar to the 'Cambridge Colour Test' the monkeys when promoted by a tone were to touch any dots that had changed from grey to another color. If they did so they were rewarded with grape juice.

After the initial round of tests, the team then injected a specially engineered virus containing the L opsin gene, the gene missing in those who are colorblind, behind the retinas of two of the color-blind monkeys.

"The virus contained genes for red pigment in cone cells—cells in the eye that respond to light and color. The virus inserted the red-pigment genes into some of the monkey's green-sensitive cone cells, causing those cells to become red sensitive."

After 20 weeks, the two monkeys were able to point out red and green. Both monkeys have retained this skill for more than two years with no apparent side effects, the researchers report in Nature.

While the usual rounds of approval for any use on humans still needs to be done, in an interview with National Geographic co-author of the study, Jay Neitz stated that "If we did this exact same thing to a human being today, I believe we would have cured their color vision."

Here, Dalton, named after the British chemist, John Dalton, who was the first to describe his own color blindness in 1794, performs the color the test after therapy.

Sources: National Geographic, Neitzvision, Nature 1 & 2

Additional reading: here's a entertaining article from 1943 written in Time Magazine about past color blindness treatment.