Mammals come in many different shades, but never the color green. Until now. Masaru Okabe, a biologist at Osaka University in Japan, engineered these green mice by injecting into mouse embryos a jellyfish gene that codes for a glowing protein called green fluorescent protein. As Okabe explains, he was trying to find a way to label mouse sperm so as to track their development, but what emerged were mice whose every cell-- except their sperm, red blood cells, and hair--glow green when exposed to blue light. The sperm lack the green glow, Okabe has found, because the protein is expressed in the cell cytoplasm, and sperm are little more than nucleus and tail. The baby mice remained green until their fur grew in, at which point only their naked paws, noses, tails, and ears retained their emerald tinge. The mice are not just a scientific curiosity; their green cells, says Okabe, could further cancer research. You could transplant green cancer cells into normal mice and track them, says Okabe, thus seeing how quickly they divide and whether they respond to treatment.