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.