Activin RIB, also known as activin receptor type-1B (ACVR1B) or ALK-4, is a type I transmembrane serine/threonine kinase receptor that is part of the TGF-β receptor superfamily. Activin binds to a type II activin receptor (ACVR2or ACVR2B) and then recruits ACVR1B[1][2][3]. Activin RIB/ALK-4 Protein, Human (HEK293, His) is produced in HEK293 cells with six C-Terminal His-tags. It consists of 103 amino acids (S24-E126) .