Fibrodysplasia ossificans progressiva is a disabling genetic disorder characterized by congenital skeletal malformations and progressive heterotopic ossification. New episodes of ossification are heralded by preosseous inflammatory lesions replete with B and T lymphocytes that overexpress bone morphogenetic protein-4. NF-kappaB is an inflammatory mediator that plays a critical role in developmental skeletogenesis and in suppression of bone morphogenetic protein-4 expression. Because of its multiple roles in inflammation, skeletogenesis, and bone morphogenetic protein-4 regulation, NF-kappaB may play an important functional role in the pathogenesis of fibrodysplasia ossificans progressiva. To clarify the potential role of NF-kappaB in the pathophysiologic features of fibrodysplasia ossificans progressiva, the role of NF-kappaB in regulating bone morphogenetic protein-4 signaling in patient-derived lymphoblastoid cell lines was examined. General NF-kappaB activity and specific NF-kappaB suppression of bone morphogenetic protein-4 expression were not altered in fibrodysplasia ossificans progressiva. In addition, despite the proximity of the gene for the p50 subunit of NF-kappaB (NFKB1 on long arm of chromosome 4) to the recently mapped locus for fibrodysplasia ossificans progressiva, a detailed linkage exclusion analysis in four multigenerational families with the disorder excluded NFKB1 as the causative gene for fibrodysplasia ossificans progressiva. These data exonerate NF-kappaB as the critical molecular and genetic pathogenic mediator in fibrodysplasia ossificans progressiva and, therefore, implicate a defect in another regulatory pathway as the cause for bone morphogenetic protein-4 overexpression in the disease.