Transforming growth factor β1 (TGF-β1) belongs to a family of homologous, disulfide-linked, homodimeric proteins. These highly pleiotropic cytokines inhibit proliferation of most cells but can promote the growth of mesenchymal cells and enhance extracellular matrix formation. The pivotal function of TGF-β1 in the immune system is to mediate immunosuppression and maintain tolerance by regulating lymphocyte proliferation, differentiation, and survival. In addition, TGF-β1 controls inflammatory responses through chemotactic attraction and activation of inflammatory cells and fibroblasts. TGF-β1 is produced by many cell types but is reported to be most abundant in mammalian platelets and bone. All three TGF-β members are synthesized as a homodimeric precursor of 390 residues, which is intracellularly processed by proteolysis into a 112 aa form. The resulting N-terminal latency associated peptide (LAP) remains non-covalently associated with the TGF-β dimer, and the complex binds to another protein called latent TGF-β-binding protein (LTBP), forming a larger complex called Large Latent Complex (LLC). The LLC is secreted into the extracellular matrix, and prevents the binding of TGF-β to its specific cell surface receptor. Several extracellular factors such as matrix metalloproteases, low pH, reactive oxygen species and thrombospondin-1 can induce release of the active mature TGF-β dimer from the inactive complex. This sophisticated mechanism of activation is important for a fine-tuning of TGF-β signaling. Human TGF-β1 is a recombinant homodimer corresponding to the fully mature form of TGF-β1 without LAP. The amino acid sequence of human TGF-β1 shares 99% identity with TGF-β1 from mouse and rat, therefore human TGF-β1 is commonly used also for mouse cell culture.