Congenital Down syndrome (DS) most frequently arises due to an error during cell division that results in three full copies of human chromosome 21 (trisomy); however, DS also occurs in people with three copies of only a part of chromosome 21. By correlating phenotype with genotype in these patients, a chromosome 21 region named the DSCR (Down syndrome critical region, 21q22.2)) has been defined. This region is responsible for many of the characteristic features of DS, such as developmental problems, B cell acute lymphoblastic leukaemia, autoimmune diseases, e.g. type 1 diabetes, coeliac disease, thyroid disease and vitiligo and Alzheimer’s disease (AD).