Genetic mechanisms and mental retardation.

The past five years have witnessed rapid and apparently relentless progress in the delineation of the genetic basis of disorders associated with mental retardation. Each gene discovery has a new story to tell but inevitably generates further questions. For the clinical geneticist and, perhaps more importantly, for patients and their families, many of these recent discoveries have yielded information which has immediate implications for diagnostic testing, family and population screening and prenatal testing. Many of the ethical issues consequent upon the rapid progress are only now being addressed. This article highlights a number of disorders whose molecular genetic basis has recently been further characterised. Brain development and maintenance of neurological networks provide the unifying theme; the genetic defects are disparate and each of their mechanisms appears to be novel.