Transforming growth factor betas (TGFbetas) were originally discovered due to their ability to promote anchorage-independent growth of rat NRK fibroblasts in the presence of TGFalpha. It is now realized that TGFbetas mediate many cell-cell interactions that occur during embryonic develop ment. Three TGFbetas have been identified in mam mals. TGFbeta1, TGFbeta2 and TGFbeta3 are each syn thesized as precursor proteins that are very similar in that each is cleaved to yield a 112 amino acid polypeptide that remains associated with the latent portion of the mol ecules. Biologically active TGFbeta requires dimerization of the monomers (usually homodimers) and release of the latent peptide portion. Overall, the mature region of the TGFbeta3 protein has approx imately 80% identity to the mature region of both TGFbeta1 and TGFbeta2. However, the NH2 terminals or precursor regions of their molecules share only 27% sequence identity.