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Normally, male squirrel monkeys can't distinguish between red and green hues-they're what is called color-blind in humans. The South American Saimiri genus lacks a gene that allows color-sensitive cells in the eye, called cones, to differentiate red and green from gray. To these animals, other colors, such as blue, brown and orange, appear faded. But Sam was one of two males in the experimental group of a groundbreaking 2009 ophthalmological study conducted at the Washington National Primate Research Center in Seattle. Husband-and-wife vision researchers Jay and Maureen Neitz injected a viral vector behind the retinas, the part of the eye that responds to color, of Sam and his simian lab partner, Dalton. The virus contained the genetic code in human eyes for red pigment, giving the monkeys an extra class of cone photoreceptor.