TGFβs mediate their activity by high affinity binding to the type II receptor (TGFβ 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 TGFβ-binding protein, designated the type I receptor. Two independent groups have recently described the cloning and sequence analysis of genes encoding TGFβ type I receptor proteins designated ALK-5 (TβR-1) and TSR-1, respectively.