A total of three members of the TGFbeta family, TGFbeta1, TGFbeta2 and TGFbeta3, have been identified in mammals. Each is synthesized as a latent precursor that is subsequently cleaved forming the 112 amino acid growth factor which becomes active upon dimerization. TGFbetas mediate their activity by high affinity binding to the type II receptor (TGFbeta RII) transmembrane protein with a cytoplasmic serine-threonine kinase domain. For signaling growth inhibition and early gene responses the type II receptor requires both its kinase activity and association with a TGFbeta-binding protein, designated the type I receptor. Two independent groups have recently described the cloning and sequence analysis of genes encoding TGFbeta type I receptor proteins designated ALK-5 (TbetaR-1) and TSR-1, respectively.