Researchers in Paris and Sydney have observed that medulloblastoma, a common brain malignancy in children, has never been reported in a person with Down syndrome of any age. Their letter to the editor, published in Neuroepidemiology online (with free access) on 17th December 2008, and in print in early 2009, describes the phenomenon as a "medical enigma". They note that closer study of Down syndrome could shed light on the development of medulloblastoma and other tumours.
D. Satgé , C.H. Rickert , A Medical Enigma: Persons with Down Syndrome Do Not Develop Medulloblastoma, Neuroepidemiology 2009;32:164