ACVR1 and Klippel-Feil syndrome: Two prominent human syndromes display vertebral fusions caused by altered BMP signaling, which in both cases are restricted to cervical vertebrae of the neck: Klippel-Feil-Syndrome (KFS) caused by loss of GDF6 or GDF3, both BMP family members (5–7), thus contrasting our data that gain of BMP signaling causes chordacentra fusions in zebrafish, and Fibrodysplasia ossificans progressiva (FOP), caused by gain of BMP signaling via a mutated, constitutively active version of the BMP type I receptor ACVR1/ALK2 (9–11), consistent with our zebrafish data.