Activin receptor-like kinase 2 (ALK2), also known as Activin A receptor type 1 (ACVR1), is a transmembrane kinase receptor for members of the transforming growth factor-β family. Wild-type ALK2/ACVR1 transduces osteogenic signaling in response to ligand binding. Fifteen years ago, a gain-of-function mutation in the ALK2/ACVR1 gene was detected in patients with the genetic disorder fibro-dysplasia ossificans progressiva, which is characterized by heterotopic ossification in soft tissues. Additional disorders, such as diffuse intrinsic pontin glioma, diffuse idiopathic skeletal hyperostosis, primary focal hyperhidrosis, and congenital heart defects, have also been found to be associated with ALK2/ACVR1. These findings further expand in vitro and in vivo model system research and promote our understanding of the molecular mechanisms of the pathogenesis and development of novel therapeutics and diagnosis for disorders associated with ALK2/ACVR1. Through aggressive efforts, some of the disorders associated with ALK2/ACVR1 will be overcome in the near future.

Bone is a unique organ because it can be experimentally induced in soft tissues by implanting a single growth factor, bone morphogenetic protein (BMP). Heterotopic bone-inducing activity was found in demineralized bone matrix in 1965. The characterization of this activity in bone enabled the purification and molecular cloning of BMPs and showed that they are members of the transforming growth factor-β (TGF-β) superfamily. Assay systems developed for this bone-inducing activity revealed the molecular mechanisms of the intracellular signaling of members of the superfamily, including BMPs. Moreover, they are being applied to elucidate molecular mechanisms and to develop novel therapeutics for a disease caused by an abnormality in BMP signaling.