US scientists have genetically modified monkeys so that they could develop the debilitating Huntington's disease.
Predictably animal lovers are upset and denounce the move as cruel.
This is perhaps the first time that primates have been genetically modified to have a human ailment.
The achievement by scientists at the National Primate Research Centre could pave the way for creating genetically modified primates with other severe degenerative brain conditions such as Parkinson's disease, Fragile X and Alzheimer's disease.
But even researchers accustomed to animal research say working with GM monkeys should only be a last resort
And the Royal Society for the Prevention of Cruelty to Animals (RSPCA) condemns the creation of primates that were designed to suffer.
"We understand that medical research is vital, but there are many different ways of carrying out research on these diseases without using primates," a spokesman said. "The animal suffering involved, in our view, would be considerable."
People are born with the faulty gene responsible for Huntigton's disease but symptoms typically do not appear until middle age.
They may include uncontrolled movements, mood swings, cognitive decline, balance problems, and eventually losing the ability to walk, talk or swallow. It affects five to 10 people in every 100,000.
There is no known treatment to halt progression of the disease, only medications to relieve symptoms. Death typically occurs 15 to 20 years after onset.
In the journal Nature Dr Anthony Chan and colleagues at Yerkes, part of Emory University, Atlanta, said one of two surviving rhesus macaque monkeys engineered to have the defective gene that causes Huntington's in humans is alread showing tell-tale symptoms aged just 10 months.
The team chose Huntington's because the hereditary disease is untreatable.
Researchers often study laboratory animals such as mice to get insights into the underlying biology of diseases and to test treatments. But when it comes to brain disease, rodents come a poor second to monkeys and other primates that are much more similar to people.
"Rodent species can capture some of the characteristics of the disease, but they have not been satisfactory in being able to really capture the essence of the disease," said Stuart Zola, head of the Yerkes centre. "Now we have a genetically modified nonhuman primate that really has captured the clinical signs that we see in patients with Huntington's disease."
The researchers also chose Huntington's as the disease for creating the genetically modified monkeys with an eye toward simplicity - because it is linked to mutations in a single gene.
Huntington's disease is one of a number of degenerative diseases marked by build up of a malformed proteins in brain cells.
In all, the Yerkes team engineered five rhesus macaque monkeys to develop the disease. The brains of one set of twins, who died a day after birth, contained clumps of the mutant protein while the lone animal, who died a month after birth, jerked involuntarily.
Recently Prof David Rubinsztein and colleagues at the University of Cambridge announced that they have identified a number of candidate drugs to investigate further which encourage cells to "eat" the malformed proteins that lead to the disease, writes Roger Highfield, Science Editor of the Telegraph newspaper, published in the UK.