TGF-beta1 (transforming growth factor beta1) is a multifunctional cytokine which regulates cellular proliferation and differentiation. It is ubiquitously expressed by many kinds of cells. Platelets express high level of TGF-beta. TGF-beta is synthesized as a large protein precursor and then secreted as a complex of TGF-beta and LAP (latency-associated peptide). TGF-beta requires activation before it binds to its receptors and exerts functions. It has been reported that LAP-TGF-beta binds to the integrins alphavbeta1, alphavbeta6, alphavbeta8 and alpha8beta1 through RGD domain. TGF-beta plays important roles in control proliferation and differentiation of epithelial cells, endothelial cells, fibroblasts, neurons, osteoclasts, and osteoblasts. TGF-beta is believed to be important in regulation of the development of Treg, Th17, and Th9 cells. Recent study has shown that LAP is an activated Treg surface marker.