Editor' Note The following paper by Dr. Anthony Graham originally belonged in the Developmental Genetics of Human Malformations issue of the Seminars in Medical Genetics [Vol 115, No. 4, Dec. 30, 2002]. It was inadvertently left out due to a processing oversight. We include it here. While it certainly supplements that issue of the Seminars series, the paper stands on its own nicely as the lead article in this issue. John C. Carey Editor

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