Patients with fibrodysplasia ossificans progressiva (FOP) develop a craniofacial phenotype characterized most notably by mandibular hypoplasia. Other less well-defined features of FOP include invol vement of the joints, muscles, and associated fascia of the head and neck. Although absent at birth, the craniofacial phnotype generally emerges during the second decade of life. The cranofacial features of FOP may provide important insight into underlying molecular signaling pathways involved in the pathogenesis of the disease.