Mice, like most mammals, normally view the world in colors limited to yellows, blues and grays, similar to what people with red-green color blindness see. By introducing a single human gene into mice, scientists endowed the animals with full-color vision. Humans and closely related primates possess an extra light-sensitive pigment permitting them to see red. (Color-seeing mammals have at least two pigments, for blue and green.) Researchers at the University of California, Santa Barbara, and their colleagues inserted the gene for this extra pigment into the mouse X chromosome. Even though the rodent brains had not evolved to use these signals, they were able to rewire themselves to handle the upgrade, correctly discriminating between colored lights to win soy-milk rewards. The investigators, who detail the work in the March 23 Science, say this finding could help explain how color vision evolved in humans.