In the EU, a rare disease is one which affects fewer than 5 people per 10 000. The number of sufferers may still be high, however, as there are some 7 000 known rare diseases. Most are caused by genetic defects, but environmental exposure during pregnancy or later in life, often in combination with genetic susceptibility, is another cause.

Some are rare forms or rare complications of common diseases. While signs may be detected at birth or in childhood, more than 50% of rare diseases appear during adulthood, and are often life-threatening or chronically debilitating. Usually there is no effective treatment, but screening for early diagnosis, followed by suitable care, can improve quality of life and life expectancy.

Lysosomal storage disorders (LSDs) are chronic, progressive, debilitating and life-threatening conditions. In patients with an attenuated phenotype, where initial disease may cause inconspicuous symptoms and signs, gradual progression to irreversible complications may be difficult to detect in time.