Genetically modified animals are a mainstay of medical research. By knocking out or adding particular genes it is possible to create models of human diseases with which to study their progress and to develop cures. While this is opposed by animal rights activists, it has public support.

The majority of GM animals created so far, however, are mice. This week brought the news of a more controversial step: the first transgenic monkeys bred to develop a human disease. The disease in question is Huntington's, a devastating neurological condition that cannot but inflict severe distress.

The research, from Yerkes National Primate Research Centre in Atlanta, was immediately attacked by animal rights campaigners. Even the RSPCA, one of the more moderate voices, condemned it because of the suffering involved.

It's hard not to feel sympathy for this position, even if, like me, you think medical experiments on animals are essential. Research with primates is emotive, not just because of their intelligence but also because they remind us of ourselves.

A photograph released by Yerkes underlined this: it showed two baby macaques clutching a blanket in an almost child-like way. While they look normal, we know they will develop a horrible disease. It is also impossible to doubt the severe impact of the modification: three of the five monkeys died in infancy.

Were this work being done to investigate a mild illness, or without regard for the animals' welfare, it would be difficult to justify. But neither criticism applies. First, Yerkes is a world leader in lab animal care. Anthony Chan, who heads the study team, explains that specialists in the human version of Huntington's, as well as animal technicians, have helped to develop a humane care plan, which will include euthanasia if suffering becomes too extreme.

Just as importantly, these experiments are not frivolous. While mouse models of Huntington's exist, rodents are not ideal for studying the human brain: the similarity of primates to Homo sapiens makes them very valuable in neurological research.

The development of a primate model is particularly important for Huntington's: it is one of the cruellest of all genetic diseases and it has

so far resisted attempts to develop effective therapies.

While people with this condition are healthy as children and young adults, at some point between their 20s and 50s they will start to suffer neurological decline. The first effects are depression, mood swings, strange behaviour and a characteristic tremor. That is followed by progressive damage to the nervous system, dementia and death. There is no cure.

What is more, Huntington's is caused by a dominant mutation in a single gene, which always triggers the disease. The precise characteristics of the mutation will affect only the age at which symptoms begin. This means that people with a parent with Huntington's know that even though they might currently be healthy, there is a 50 per cent chance that they have inherited the disease themselves.

This rare example of true genetic determinism hangs over whole families like a death sentence. In the absence of good therapies, many people who know they are at risk choose not to be tested for the mutant gene.

This explains why Dr Chan says it would be unethical not to apply his team's expertise in genetic modification to Huntington's. For all the understandable qualms people have about primate suffering, he is right. While it is impossible to be certain that these transgenic animals will lead to a cure, they will bring a new dimension to research.

It would probably be proper to create GM monkeys to investigate diseases that are less serious or intractable than Huntington's, if the work is necessary. For this peculiarly unpleasant condition, the ethical balance between animal suffering and medical opportunity is clearer still.

Mark Henderson is Science Editor of The Times

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