In a controversial achievement, Japanese scientists announced on today they had created the world's first transgenic primates, breeding monkeys with a gene that made the animals' skin glow a fluorescent green.
The exploit opens up exciting prospects for medical researchers, they said.
It could eventually lead to lab monkeys that replicate some of humanity's most devastating diseases, providing a new model for exploring how these disorders are caused and how they may be cured.
"Great advances in pre-clinical research can be expected using these models," the team said.
But other voices warned of a potential ethics storm, brewed by fears that technology used on our closest animal relatives could be turned to create genetically engineered humans.
In a study published in the British journal Nature, a team led by Erika Sasaki of the Central Institute for Experimental Animals at Keio University reported on experiments on common marmosets (Callithrix jacchus), a small monkey native to Brazil.
They introduced a foreign gene, tucked inside a virus, into marmoset embryos that were then nurtured in a bath of sucrose.