Mechanical force is seen today as a key component of molecular processes in cells: forces can be signals as in touch receptors, products as in muscle action, and substrates as in the matrix surrounding moving cells . This view of molecular processes is the result of a series of ground-breaking investigations that have become possible only recently and is rapidly turning into a new field, mechanobiology (see recent review). A collaboration with V. Vogel and coworker (U. Washington, Seattle) has investigated structural changes accompanying stretch-induced unfolding events in type III fibronectin, a protein of the extracellular matrix, explaining the design of these proteins.