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In the article “Technical standards and guidelines: Molecular genetic testing for ultra-rare disorders” in the October 2005 issue of Genetics in Medicine, the definition of a rare disorder, rather than an ultra-rare disorder, was used in section URD 2.1 on page 572. The authors would like to clarify these definitions.

A rare disorder, as defined by the Orphan Drug Act of 1983, affects populations smaller than 200,000 individuals in the USA. An ultra-rare disorder, as defined by common usage, is a disease occurring in less than 2,000 individuals in the USA.