TGF-β1 (transforming growth factor β1) is a multifunctional cytokine which regulates cellular proliferation and differentiation. It is ubiquitously expressed by many types of cells. Platelets express high level of TGF-β. TGF-β is synthesized as a large protein precursor and then secreted as a complex of TGF-β and LAP (latency-associated peptide), in which LAP noncovalently associates with the dimeric mature TGF-β to prevent its activity. TGF-β requires activation before it binds to its receptors and exerts functions. It has been reported that LAP-TGF-β binds to the integrins αvβ1, αvβ6, αvβ8, and α8β1 through RGD domain. TGF-β plays an important role in controlling proliferation and differentiation of epithelial cells, endothelial cells, fibroblasts, neurons, osteoclasts, and osteoblasts. TGF-β is believed to be important in the regulation of the development of Treg, Th17, and Th9 cells. A study has shown that LAP is an activated Treg surface marker.