Transforming growth factor-β (TGF-β) family members,which include TGF-βs,activins and bone morphogenetic proteins,are pleiotropic cytokines that elicit cell type-specific effects in a highly context-dependent manner in many different tissues.These secreted protein ligands signal via single-transmembrane Type Ⅰ and Type Ⅱ serine/threonine kinase receptors and intracellular SMAD transcription factors.Deregulation in signaling has been implicated in a broad array of diseases,and implicate the need for intricate fine tuning in cellular signaling responses.One important emerging mechanism by which TGF-β family receptor signaling intensity,duration,specificity and diversity are regulated and/or mediated is through cell surface co-receptors.Here,we provide an overview of the co-receptors that have been identified for TGF-β family members.While some appear to be specific to TGF-β family members,others are shared with other pathways and provide possible ways for signal integration.This review focuses on novel functions of TGF-β family co-receptors,which continue to be discovered.