Clone CH6-17E5.1 reacts with the N-terminal latency-associated peptide (LAP) of the transforming growth factor β1 (TGF-β1) dimer. 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. TGF-β1 is produced by many cell types, but is reported to be most abundant in mammalian platelets and bone. It is secreted predominantly as an inactive latent complex. After proteolytical processing of the TGF-β1 precursor, the resulting N-terminal latency-associated peptide (LAP) remains non-covalently associated with the TGF-β1 dimer. Mature and biologically active TGF-β1 can be released from the complex by action of proteases and/or conformational changes. LAP-TGF-β1 was recently described to be expressed on the cell surface of activated human regulatory T cells.