A potent peptide inhibitor (SuperSog) of TGF-.beta. family growth factor signaling, peptide variants thereof and nucleotide coding sequences therefore are provided by the invention. The Super-Sog peptide comprises a fragment of the Drosophilia short gastrulation (Sog) gene which includes the CR-1 cysteine-rich repeat of Sog. Methods and compositions for use of Super-Sog in therapeutic and diagnostic applications are also provided.