Thymosin beta 4 is a small, 5-kDa protein with a diverse range of activities, including its function as an actin monomer sequestering protein, an antiinflammatory agent, and an inhibitor of bone marrow stem cell proliferation. Only the effects of thymosin beta 4 on the actin cytoskeleton have an explanation based on identified molecular interactions. Thymosin beta 4 is largely unfolded or perhaps completely unfolded in solution. Based on the paradigm introduced by Wright and Dyson (1999) that unfolded proteins may have multiple functions based on their ability to recognize numerous ligands, the flexible structure of thymosin beta 4 may facilitate the recognition of a variety of molecular targets, thus explaining the plethora of functions attributed to thymosin beta 4. Furthermore, if multiple ligands bind to thymosin beta 4, then it is possible that thymosin beta 4 has a unique integrative function that links the actin cytoskeleton to important immune and cell growth-signaling cascades.