Three members of the TGF beta family, TGF beta 1, TGF beta 2 and TGF beta 3, are known in mammals. TGF betas mediate their activity by high affinity binding to the type 2 receptor, which requires both its kinase activity and association with members of a series of related type 1 receptors called activin receptor like kinase (ALK) 1 to 6. Of these, only ALK5 has been shown to represent a functional TGF beta type 1 receptor, but only ALK2 and ALK4 bind activin with high affinity. TGF beta R3 binds TGF beta and retains TGF beta for presentation to the signaling receptors.