Legislation exists to ensure that research into drug development for rare diseases is encouraged. But how sure can we be that a rare disease is, in fact, rare, and what happens when the real prevalence is found to be higher, or lower, than official estimates suggest? Laura Spinney reports.

On June, 30, 2010, an All Party Parliamentary Group on Huntington’s disease will be launched in the UK. The group’s first task will be to investigate the true prevalence of Huntington’s disease, because a discrepancy exists between the prevalence estimate that the UK government has been working with and that suggested by the number of people seeking advice from the UK-based Huntington’s Disease Association.“The figure that is generally reported is seven symptomatic people per 100 000”, says Michael Rawlins, chairman of the National Institute for Health and Clinical Excellence for England and Wales.“The Huntington’s Disease Association has 6376 [symptomatic people] on its books in England, which works out at 12· 4 per 100 000.” In other words, if these figures accurately reflect the UK situation, the prevalence of Huntington’s disease has been underestimated by 80%. Because the figures from the Huntington’s Disease Association are themselves almost certainly an underestimate, Rawlins says it is reasonable to suggest that Huntington’s disease might be twice as prevalent in the UK as it was previously thought to be. The prevalence figures from the Huntington’s Disease Association have yet to be validated, and Rawlins—along with Sarah Tabrizi of University College London’s Institute of Neurology, London, UK, who leads a research programme on Huntington’s disease, and Stephen Evans, an epidemiologist at the London School of Hygiene and Tropical Medicine—is now designing a study to do just that. Their research will involve combing general practice databases for diagnoses of Huntington’s