Tenascin-C is a member of a small family of closely related extracellular matrix molecules. The protein is highly conserved during evolution and is expressed in a variety of tissues. In the nervous systems of rodents and chickens, tenascin-C is predominantly expressed at early developmental ages and has been hypothesized to be functionally involved in different steps of neural development. For example, tenascin-C has been implicated in synaptogenesis, migration of different neural cell types, axonal growth in the developing and lesioned nervous system, and in the formation and maintenance of discrete anatomical boundaries. Experiments with monoclonal anti-tenascin-C antibodies or recombinantly expressed tenascin-C fragments have localized different functions of tenascin-C to different domains of the protein. Functional diversity of tenascin-C in vitro might additionally relate to a cell type-specific expression of different receptor(s) and/or to differences in intracellular signal transduction pathways. Surprisingly, mutant mice reported to completely lack expression of tenascin-C develop normally and reveal no detectable phenotype, suggesting that tenascin-C does not play a critical role in the intact organism. However, residual expression of an abnormal fragment of the tenascin-C protein has recently been detected in this mutant.