Involved in dorsal-ventral patterning, permitting peak BMP signaling by antagonizing the residual anti-BMP activity of the cleavage products of chrd. Functions to promote the formation of ventral mesoderm by increasing the activity of bmp7 and other BMPS. Seems to antagonize BMP signaling by forming ternary complexes with chrd and BMPs, thereby preventing BMPs from binding to their receptors. In addition to the anti-BMP function, also has pro-BMP activity, partly mediated by cleavage and degradation of chrd, which releases BMPs from ternary complexes. May be an important modulator of BMP-regulated cartilage development and chondrocyte differentiation. Binds directly to bmp2, bmp4 and bmp7 and can form a ternary complex with bmps and chordin, thus preventing the binding of bmps to their cell surface receptors. Maternal transcripts are distributed throughout animal half of the embryo during cleavage stages. At the late gastrula stage, maternal transcripts decrease and zygotic transcripts appear specifically in the ventral region of the embryo. After neurulation, expressed ventrally in the closed blastopore slit and the neural tube. At the tailbud stage, detected in the postanal region, heart and dorsal eye. The N-terminal domain is sufficient to interact with bmp4. Knockdown of tsg and bmp7 results in the siren phenotype with loss of posteroventral cell fates associated with decreased BMP activity. Belongs to the twisted gastrulation protein family.