All male squirrel monkeys are color-blind. Unlike their better-endowed female counterparts, these fellows lack a crucial visual pigment called L-opsin that let them distinguish reds from greens. For two lucky male squirrel monkeys, however, color-blindness is now a thing of the past. Reasoning that the female version of L-opsin might also do the trick for males, researchers from the University of Washington used a harmless virus to carry the pigment gene behind the monkeys' retinas.

The adult monkeys, nicknamed Dalton and Sam, had already been trained to recognize specific patterns of blue and yellow dots against a gray screen; now the scientists checked to see if they could also see patterns of red and green dots. Success came about 5 months later, when the duo began paying attention to the new colors. It's now been two years since the monkeys first experienced the full color spectrum, and there have been no further changes in their vision and no noticeable side effects. Researchers hope this technique will lead to treatments for color blindness in humans.