The creation of monkeys that 'glow in the dark' has sparked an ethical storm.
The designer marmosets carry a gene that causes their skin, hair roots and blood to glow green under ultraviolet light.
What makes this a world first is that scientists were able to show that the monkeys can pass on the gene to other generations.
Enlarge Fluorescent: Kei the monkey has been genetically engineered so the soles of his feet glow green in the dark (inset)
Fluorescent: Monkeys Kei and Kou have been genetically engineered so the soles of his feet glow green in the dark (inset)
This raises the prospect of deliberately breeding monkey colonies with the genetic defects that cause incurable diseases.
Scientists say the method they used to create the fluorescent monkey could help in the search for cures for devastating diseases such as Parkinson's and MS.
Although much information has been obtained from manipulating mice in this way, it is argued that monkeys would make better models for human diseases.
But others say the experiments leave the door open to a 'brave new world' of designer babies, genetically tweaked to be as perfect as possible. There are also concerns about deliberately inflicting disease on generation after generation of animal.
The researchers, from Central Institute for Experimental Animals in Japan, used viruses to carry a gene for a fluorescent protein into more than 90 marmoset embryos.
Enlarge Controversial: The success of the genetic engineering has opened up the debate over whether monkeys should be used for medical experiments
Controversial: The success of the genetic engineering has opened up the debate over whether monkeys should be used for medical experiments
Five designer monkeys were later born, including twins Kei and Kou, after 'keikou', the Japanese for fluorescence.
The green glow produced when a UV light was shone on the creatures confirmed the fluorescent protein was being made in their tissues, the journal Nature reports.
Dr Kieran Breen, of the Parkinson's Disease Society, described the work as 'potentially very exciting'. He added: 'At the moment we use mice with mutant genes that are associated with Parkinson's to search for new drugs to treat the condition.
Because non-human primates are much closer to humans than mice genetically, the successful creation of transgenic marmosets means we will have a new animal model to work with.'
But others raised concerns. Michelle Thew, of the British Union for the Abolition of Vivisection, said: 'Instead of inflicting great suffering on other species, the research industry should be investing its substantial intellect, ingenuity and resources into finding more appropriate and humane methods to fight human diseases.'
Dr David King, of the group Human Genetics Alert, said the work raised the prospect of babies being genetically engineered. 'I'm worried these steps are being taken without any overall public discussion about whether we want to go down that road,' he said.
