TGF beta signalling pathways apart from receptors, their ligands and intracellular effectors consist of proteins capable to bind ligands which are not classical receptors. They do not transmit the signal directly into the cell but by their interaction with TGF beta family factors and TGF beta type I and II receptors can influence TGF beta cascade activity. There are three proteins characterised by such features i.e., betaglycan (TGF beta RIII), endoglin (CD105) and antigen CD109 which are commonly called as TGF beta accessory/auxiliary receptors or TGF beta type III receptors. Their key function is to modulate accessibility of ligands to TGF beta type I and II receptors. The role of TGF beta accessory receptors has been well documented for signal transduction via TGF beta isoforms--TGF beta 1, TGF beta 2 and TGF beta 3. Each TGF beta accessory receptor has different TGF beta isoforms binding specificity and cellular response character. TGF beta cascade is the only known cellular signalling pathway for which accessory receptors are identified until now.