In one type of squirrel monkey, the males lack a visual pigment called L-opsin. Its absence renders the monkeys color-blind, unable to distinguish reds and green. Most of the females, on the other hand, see in full color. So the scientists got to wondering: what would happen if they gave a boy squirrel monkey the same opsin that girls have.
Using a harmless virus, the scientists introduced the pigment gene into the eyes of color-blind adults. Lo and behold, about a month later, the monkeys with the new L-opsin gene were able to see hues they’d never seen before.