Recently, studies in the usually disparate fields of human genetics and developmental neurobiology have converged to reveal that some types of human mental retardation and brain malformations are due to mutations that affect the neural cell adhesion molecule LI. LI has a very complex biology, interacting with a variety of ligands, and functioning in migration of neurons and growth of axons. Over the past few years, it has also become clear that LI is able to influence intracellular second messengers. The identification of a number of different mutations in LI, some of which alter the extracellular portion of the molecule, and others that change only the cytoplasmic tail, confirm that LI is a crucial player in normal brain development. The information gained from genetic analysis of human LI is giving new insights into how LI functions in the formation of major axon pathways, but it also raises unanticipated questions about how LI participates in the development of cortical and ventricular systems.
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