As an important regulator of TGF-β1 activity, Latency-Associated Peptide (LAP) is secreted as a complex with TGF-β1. LAP and TGF-β1 are synthesized as a single propeptide precursor of 390 amino acids with an N-terminal signal peptide of 29 amino acids, a 249 amino acids pro-region (LAP), and a 112 amino acid C-terminal region (TGF-β1). Both LAP and TGF-β1 exist as homodimers in circulation, but the disulfide linked homodimers of LAP and TGF-β1 remain non-covalently associated, forming the small latent TGF-β complex (SLC, 100 kD). The large latent TGF-β complex (LLC, 235-260 kD) contains a third component, the latent TGF-β binding protein (LTBP), which is linked to LAP by a single disulfide bond. LTBP does not confer latency but is for efficient secretion of the complex to extracellular sites. Previously, LAP was presumed to function only as a sequestering agent for TGF-β1. However, recent studies showed that LAP has in vitro or in vivo functions independent of TGF-β1. LAP can induce epithelial cell migration and promote chemotaxis of monocytes and block inflammation. In vivo, LAP enhances hepatocyte regeneration and reduces fibrosis. LAP is also a surface marker of activated regulatory T cells.