Human chromosome 21 is the smallest human autosome and many important genetic/familial disorders map to this chromosome, e.g., familial amyotrophic lateral sclerosis (FALS), Down syndrome, Alzheimer's disease and some cases of Ewings sarcoma. Hence, the identification of genes localised to this chromosome and studies on their normal biological function and their role in disease is gaining momentum. The use of animal models to generate gain- and loss-of-function mutations is an important element of these studies on functionality/pathology and has yielded powerful insights. However, no animal model has yet been generated that exactly models any of the disorders associated with this chromosome. The major utility of the animal models has been to illuminate the biological functions of genes and the causation of pathophysiology of diseases associated with genes on this chromosome.