Fibrodysplasia ossificans progressiva is a heritable disorder of mesenchymal cell differentiation in which heterotopic bone develops in the connective tissue surrounding skeletal muscle. We have attempted to elucidate the basic cellular defect in this condition from studies of fibroblasts in culture, and tissue implants in receptive animal hosts. Several studies have been started to establish a course of effective medical intervention in fibrodysplasia ossificans progressiva (FOP).