In this undated photo released Thursday by Tokyo-based NPG Nature Asia-Pacific, the feet of what is claimed to be the world's first transgenic marmoset infant are shown glowing under ultraviolet light. (E. Sasaki et al/NPG Nature Asia-Pacific/Associated Press)

Scientists gave marmosets a gene that made their feet glow green, and one of the animals passed it along to its offspring — the first time that an added gene has been inherited by a monkey.

It was a milestone, experts said, that should make it easier to produce animals with versions of human disease for medical research.

Animals that get added genetic material are called transgenic. While researchers have long created transgenic mice and other animals by giving them extra genetic material, monkeys offer a promising avenue for medical studies because of their similarity to humans.

Researchers have added genes to rhesus macaques before by injecting embryos, but the new work is the first documentation that such genes can be passed along to future generations of monkeys.

That's important because it opens the door to creating colonies of transgenic monkeys by breeding, which would be far simpler than the cumbersome process of making each animal from scratch by injecting a gene into an embryo.

The work is reported in Thursday's issue of the journal Nature by scientists at the Central Institute for Experimental Animals in Kawasaki, Japan, and elsewhere in that country.

The researchers plan to use transgenic marmosets to study such conditions as Parkinson's disease and Lou Gehrig's disease, or ALS.

Anthony Chan of the Emory University School of Medicine in Atlanta said the result boosts his confidence that his transgenic macaques will also pass along their added genes to offspring, once they become old enough to reproduce.

For the study, the researchers used a gene that makes tissues glow under ultraviolet light, as an easy way to see where the gene is present. They put the gene in a virus that would insert it into the DNA of cells, and then injected the virus into marmoset embryos.

From these embryos, five healthy marmosets were born. All showed evidence of having inherited the gene.

Later, one of those animals fathered a male by test-tube fertilization. The gene was shown to be active in the offspring's skin.

"The birth of this transgenic marmoset baby is undoubtedly a milestone," Gerald Schatten of the University of Pittsburgh School of Medicine and a colleague wrote in a Nature commentary.

Transgenic marmosets could be useful for studying infectious diseases, immunology and neurological disorders as well as some genetic disorders like muscular dystrophy, they wrote. But marmoset biology differs enough from humans to prevent study of other disorders like AIDS and tuberculosis, which can be approached instead through other monkeys that are more closely related to humans, they wrote.