The transforming growth factor β (TGF-β) family of cytokines plays diverse and important roles in growth, development, and differentiation. Three high affinity TGF-β receptor types have been characterized. Both type I and type II receptors are type I transmembrane proteins which contain a Ser/Thr-kinase in their cytosolic domains. Type III receptor, also known as betaglycan, has no cytosolic signaling motif and functions in the presentation of TGF-β to the type I and type II receptors. The cytosolic domain kinase of the type I receptor is thought to be transphosphorylated by the corresponding kinase of the type II receptor, leading to activation of downstream substrates. Ser/Thr receptor associated protein (STRAP) contains six WD domains, which are characteristic of signal transduction proteins. STRAP can associate with both TGF-βI and TGF-βII receptors and overexpression of STRAP leads to inhibition of TGF-β-dependent transcriptional activation. STRAP is expressed ubiquitously in species ranging from yeast to human. Thus, STRAP may be an important element of TGF-β signal transduction pathways in a variety of tissues and species.