activin activin receptor aggrecan alternative splicing bambi biglycan bone morphogenetic protein bone morphogenetic protein receptors cell cycle cell signaling chondroitin sulfate chordin collagen connective tissue cripto dermatan sulfate digital object identifier ensembl entrez epidermal growth factor receptor extracellular matrix fibril fibrillogenesis fibronectin follistatin gene gene duplication glycoprotein glycosaminoglycan growth differentiation factor homologene i-smad inhibin mouse genome informatics nodal protein data bank proteoglycan r-smad signal transduction tgf beta tgf beta receptors tgf beta signaling pathway thrombospondin transforming growth factor beta superfamily versican