This spring, news of a biological breakthrough arrived in the form of baby marmosets whose feet glowed green under ultraviolet light. Researchers at the Central Institute for Experimental Animals in Kawasaki, Japan, had genetically engineered the monkeys to incorporate a gene, derived from jellyfish, that produces green fluorescent protein. It was the first time scientists had added a gene to a primate in such a way that a new trait could be passed to a second generation.

The feat heralds an exciting possibility: if the genes associated with some cases of human illnesses such as Huntington's disease, Parkinson's disease, amyotrophic lateral sclerosis (ALS), and Alzheimer's disease were introduced into primates, colonies of the genetically altered animals could be used to test therapies for these disorders. This would probably be far more effective than studying the effects of the genes in, say, mice or rats, because primates' brains are much closer to humans' in terms of complex motor functions and cognition. "We've been waiting a long time for [disease] models like these," says John Morrison, a professor of neuroscience at Mount Sinai School of Medicine in New York.