The latent transforming growth factor-β (TGF-β) 1 binding proteins (LTBP-1-4) are secreted multidomain glycoproteins, three of which are central to TGF-β regulation (1, 2). LTBP-1 is a component of the latent TGF-β complex that consists of TGF-β, the TGF-β propeptide, and LTBP (Fig. 1) (3). TGF-β is synthesized as a homodimeric proprotein, and the dimeric propeptide is cleaved intracellularly from the growth factor (4). Unlike most propeptides that have little affinity for the mature protein, the TGF-β propeptide strongly binds to TGF-β, and the proteins are secreted in complex (5). In this small latent complex (SLC), TGF-β cannot bind to its surface receptors. Therefore, the propeptide is called the latency-associated protein (LAP). The dissociation or activation 2 of TGF-β from LAP is a critical regulatory event as all TGF-β is secreted in a latent form. The LAP dimer is usually disulfide-bound to a second gene product, LTBP, and the trimolecular aggregate is called the large latent complex (LLC).