Bone morphogenetic protein 4 (BMP-4) is a member of the highly conserved transforming growth factor β (TGF-β) superfamily. Mature BMP-4 is a disulfide-linked homodimeric protein consisting of two 116-amino-acid residue subunits, and is generated by the proteolytic removal of the signal peptide and propeptides (Xiao et al.). BMP-4 binds to type I and type II receptors on cells. This binding results in the phosphorylation of receptor 1, which in turn results in the phosphorylation of Smad proteins, which then go on to act as transcription factors (Zhang et al.). BMPs have been shown to be key regulators of embryogenesis and are known to play a role in the growth and differentiation of various cell types, including embryonic stem (ES) cells, induced pluripotent stem (iPS) cells, mesenchymal cells, epithelial cells, hematopoietic cells, and neuronal cells (Chadwick et al.; Graham et al.; Jones et al.; Lengerke et al.; Zhang et al.).