Tenascin is believed to be an important extracellular matrix protein involved in regulating numerous developmental processes, such as morphogenetic cell migration and organogenesis. This function was implied by its tissue distribution and regulated expression during embryogenesis. However, such an important function was questioned recently, since mice lacking a functional tenascin gene apparently developed normally. To us, this is not conclusive evidence that tenascin has no function, for we believe that the loss of tenascin could be compensated for. Since in several other cases compensation occurs by other members of a gene family, we started to investigate the family of tenascin-like genes and we review in this article the structures of the original tenascin/cytotactin (tenascin-C), restrictin/J-160/180 (tenascin-R), and the tenascin-like gene present in the major histocompatibility complex class III locus (tenascin-X). Furthermore, we present evidence for the likely existence of even more members of this tenascin family.