Publisher Summary The hexabrachion is a large oligomeric glycoprotein of the extracellular matrix (ECM). It is synthesized at very specific times and locations during embryonic development, it is absent or restricted in most adult tissues, and it is prominently expressed in a variety of tumors. The protein is present in some apparently stable locations, but is especially prominent in states of development and growth, both embryonic and cancerous. The hexabrachion protein has been discovered independently by several laboratories, each discovery showing a different context of its biology, and each laboratory giving it a different name. The various names given to the protein are listed in the chapter in tabulated form. The survey of tissues showing high concentrations of tenascin provides intriguing clues about its possible functions. Roles in stimulating cell growth, in cell migration, and in cell adhesion have been postulated for some of these locations. Other tissues suggest that tenascin has little direct interaction with most cells, but may play a role in structuring the fibers of the ECM.