Down syndrome is a developmental disorder caused by the presence of an extra copy of chromosome 21. This extra genetic material causes changes in the production of various proteins, such as enzymes and cell adhesion molecules, which can disrupt metabolic pathways and cause defects in organ development. Modern medical advances and the development of early interventions have reduced the impact of Down syndrome, and as a result, individuals with Down syndrome can now live longer and have fewer physical and intellectual difficulties. Down Syndrome, Second Edition explains this genetic disease, its history and characteristics, and what scientists are doing to study it.