With just two photoreceptors to our three, mice normally view the world as a person with red-green color blindness would. But last week, scientists reported that minor genetic tweaking could endow mice with human color vision. Researchers from Johns Hopkins University and the University of California, Santa Barbara, genetically engineered mice to express a third, humanlike photoreceptor. The mice were able to distinguish colors they previously could not see, despite the fact that only their retinas had been tinkered with--the wiring in their brains, which had evolved to use only two photoreceptors, was able to quickly adapt to take advantage of three. "Sensory systems are quite plastic," says Jeremy Nathans, a researcher at Johns Hopkins who led the project.

So if scientists can create X-mice so easily, can they do the same for humans? For example, could they engineer a fourth photoreceptor that would allow us to peer through the night with tigerlike clarity or see ultraviolet radiation like bumblebees? Theoretically, the answer is yes, says Nathans. But practical restraints make it unlikely. "We wouldn't be able to see UV light because the lens in eye is a UV cut-off filter," he says. And background thermal radiation would create a problem at the other end of the spectrum, he adds.

However, some people may actually have a fourth photoreceptor that detects light within the visible range at a slightly different wavelength than the other three. Nathans, who in the 1980s uncovered the structure of the light-sensing proteins in our three types of photoreceptors, says that some people have a fourth variant. While it doesn't give them night vision, it might provide a richer color experience, allowing them to distinguish differences in colors that others cannot.