The L1 adhesion molecule is a 200-220kD membrane glycoprotein of the immunoglobulin superfamily. Initially studied in the nervous system, L1 is involved in important processes such as neuron-neuron adhesion and migration, axonal outgrowth and fasciculation, myelination, and synaptic plasticity. Mutations in the L1 gene product have been shown to cause abnormal brain development. Outside the nervous system, L1 is expressed in hematopoietic and certain epithelial cells in humans. L1 is also active in the mitogen-activated protein kinase (MAPK) signaling cascade.