The evidence for a functional role of cytotactin/tenascin (CT/TN) in several developmental processes with particular emphasis on those related to the nervous system are reviewed. CT/TN gene regulation, as well as the growing family of TN-related genes, is briefly discussed. Finally, I will explore the possibility that gene disruption experiments may not provide a unique resolution of gene function that was initially predicted when it became possible to manipulate mammalian genes with relative ease. It may be necessary to reevaluate the idea that these knockout experiments provide a definitive answer to the question of the function of a gene product. Cautious interpretation should be exercised in view of the large number of variables that can operate during embryogenesis, the existence of compensatory mechanisms during regulative development, and the pleiotropy resulting from mutation or deletion of a single gene. The reader should consider this a modest proposal, not a dogmatic one.