Transforming growth factor-β (TGF-β) family, including TGF-β, activin, Nodal, bone morphogenetic proteins (BMPs) and others, play vital roles in diverse cellular processes, including cell proliferation, differentiation, apoptosis, cell plasticity and migration. The type I receptor serine/threonine kinases (RSTKs) are also known as activin receptor-like kinases (ALKs) for which a systematic nomenclature has been proposed (ALK1-7) Its dysfunctions can result in various kinds of diseases, such as cancer and tissue fibrosis. Ligand binding leads to formation of the receptor heterocomplex, in which TGF-βRII phosphorylates threonine and serine residues  of TGF-βRI and thus activates TGF-βRI. The activated TGF-βRI recruits and phosphorylates a subset of SMAD proteins (SMAD 2/3) which are then translocated to the nucleus where they form transcription complexes with DNA binding factors and co-activators/co-repressors to regulate transcription of the target genes[1]. In normal cells, TGF-β, acting through its signaling pathway, stops the cell cycle at the G1 stage to stop proliferation, induce differentiation, or promote apoptosis. When a cell is transformed into a cancer cell, parts of the TGF-β signaling pathway are mutated, and TGF-β no longer controls the cell. These cancer cells proliferate[2].
Activins are dimeric growth and differentiation factors which belong to the transforming growth factor-beta (TGF-beta) superfamily of structurally related signaling proteins. Activins signal through a heteromeric complex of receptor serine kinases which include type I and type II receptors, and transduce signals through Smad-dependent and independent mechanisms. Type I receptors are essential for signaling; and type II receptors are required for binding ligands and for expression of type I receptors. Type I and II receptors form a stable complex after ligand binding, resulting in phosphorylation of type I receptors by type II receptors. Type II receptors are considered to be constitutively active kinases[3],[4].

RSTKs listed: BMP-ALK, TGF-βR