What exactly do we mean by a rare disease (RD), and why are they important for paediatricians? The definition used for public health purposes in Europe is that an RD is one which affects fewer than one citizen in 2000; in the USA it is fewer than one in 1250.1 There are estimated to be between 6000 and 8000 known RDs in the world, many of which are predominantly paediatric disorders. About 75% of the diseases meeting the criteria for RD affect children, and about 30% of all patients with RDs die before their fifth birthday.2 Approximately 80% of RDs have a defined genetic basis.2 The true incidence and prevalence of individual RDs are often unclear. When the condition is fatal in childhood or early adult life, the prevalence in the population (table 1) will be well below the birth incidence, and will not reflect the gene frequency. Thus, to take the familiar example of cystic fibrosis, the birth incidence in the UK is about one in 2500 live births, but the population prevalence is given as about one in 8000.

Although individually rare, the cumulative burden of RDs is significant. RDs may affect as many as 30 million Europeans, with at least 3 million in the UK and 4 million in Germany, and this statistic has not been lost on the European Union (EU). A Committee of experts on rare diseases (EUCERD) was set up in November 2009 to assist and advise the European Commission, including, inter alia, drawing …