Advances in molecular biology promise to enhance our understanding of the genetic basis of many disorders associated with developmental disabilities. The development of new technologies, such as polymerase chain reaction (PCR) and automated deoxyribonucleic acid (DNA) sequencing, and the extraordinary outcome of the Human Genome Project in identifying ~35,000 genes in the human genome will allow the study of many of these disorders in more detail at the molecular level. Ultimately, understanding the molecular basis of these disorders may result in improved therapies directed toward the basic molecular defects that cause them.