TGF-beta-1 is a multifunctional cytokine that belongs to a superfamily of structurally related regulatory proteins, which includes three mammalian TGF-beta isoforms (TGF-beta-1, -beta-2, and -beta-3), activin/inhibins and bone morphogenetic proteins. The most abundant isoform, TGF-beta-1, is a 25 kDa homodimer composed of two 12.5 kDa subunits joined by disulfide bonds. TGF-beta-1 is a highly conserved molecule - the amino acid sequence between human and mouse differ only by one residue. Although originally defined by its ability to cause anchorage independent cell growth and changes in cell morphology of rat fibroblasts, subsequent research has revealed that TGF-beta is actually a major growth inhibitor for most cell types. It is produced by a wide variety of cell and tissue types during all stages of cell differentiation. TGF-beta-1 sources include platelets, bone and soft tissues such as placenta and kidneys.