There has been an increasing recognition that rare diseases represent a substantial global health burden. In the United States, rare or orphan disease is defined as a condition affecting less than 200, 000 persons, which is approximately 63 per 100, 000 persons (1, 2). In Europe, a disorder is defined as rare when it affects less than 1 in 2, 000 (approximately 50 per 100, 000) persons. Because there are more than 6, 000 rare diseases, it is estimated that 8% of the general population are affected by these rare disorders, and many of them remain undiagnosed (1, 2).
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