Microcephaly: a review of genetic implications in its causation.

Microcephaly is a clinical sign rather than a nosological entity. It may even represent the extreme of normal variation. In pathological cases, it is always caused by an interruption of the neurobiologic processes of induction and cellular migration, or by a catastrophic insult to the central nervous system. The prime cause of this may be environmental or genetic. There is strong evidence for genetic heterogeneity, even among cases of 'true' or 'primary' microcephaly. Various taxonomies for the classification of microcephaly are discussed, taking into account environmental causation and various genetic mechanisms.