Everyday tasks such as telling whether a steak is well done or spotting sunburn can be next to impossible for people with red–green colour blindness. Now Jay and Maureen Neitz, a husband-and-wife research team at the Eye Institute of the University of Washington in Seattle, and their colleagues have successfully used gene therapy to fix the condition in adult monkeys, bringing new colour to their world.

Colour blindness affects about 1 in 12 men and 1 in 230 women. Red–green colour blindness is by far the most common type, and is typically caused by mutations in genes on the X chromosome that inactivate the light-absorbing pigments of the retina. One type of mutation, in the long-wavelength (L) opsin gene, results in an absence of red photopigment, making both red and green objects appear grey.

Interestingly, some male squirrel monkeys (Saimiri sciureus) also lack red photopigment in their eyes. Jay and Maureen Neitz wondered whether introducing the gene for the missing photopigment by gene therapy could provide a basis for full colour vision in these monkeys.

But the project was something of a leap of faith. “The word on the scientific street was that it wouldn't be possible to add new sensations to adults,” says Maureen. Experiments conducted in the 1970s had indicated that there is a critical period for the development of many of the brain's capabilities, including vision, and that new ones could not be added later in life. “It seemed unlikely to work, but would be so amazing if it did that we had to try,” she adds.

It took close to a decade for the Neitzes and their collaborators to get all of the facets of the experiment working, such as selecting the appropriate vector to carry the L-opsin gene inside the eye and making sure it was active in the cells of the retina. One crucial component was developing a way to determine which colours the monkeys could discriminate. In the study described on page 784, the researchers used a computer-based test in which two monkeys, Dalton and Sam, who had been colour-blind since birth, were shown a screen with a patch of coloured dots — blue, yellow, red or green — against a background of grey dots. The animals had to touch the screen in the position of the colour, and for every accurate answer received a squirt of grape juice.

The colour-blind monkeys could readily identify yellow and blue patches, but when the colour was a hue of red or green they hesitated, eventually making a tentative guess as to where to touch the screen. “This is similar to how humans behave. We tend to guess too,” says Jay.

But five months after having their retinas injected with a virus carrying the human L-opsin gene, the monkeys were suddenly able to spot red and green patches, no matter how faint. “They seemed to enjoy doing the tests more after that,” says Jay.

The work shows that the adult nervous system can pick up new faculties. It also offers hope for treating colour blindness, and other diseases that affect the retina, in humans. “We receive a lot of e-mails from people who are interested in participating in pilot studies and my heart goes out to all of them,” says Jay. But, he cautions, that day is still in the future. However, he adds that he, his wife and others are forging ahead with making the technique safe for humans.