Tenascin, also known as hexabrachion and cytotactin, is an extracellular matrix protein with a spatially and temporally restricted tissue distribution. It is a hexameric, multidomain protein with disulfide linked subunits of 190 to 240 kD, originally characterized as 'myotendinous antigen.' In the embryo it is present in dense mesenchyme surrounding developing epithelia and in developing cartilage and bone. In the adult, tenascin remains present in tendons and myotendinous junctions in the perichondrium and periosteum, as well as in smooth muscle.