GDF3 and Klippel-Feil syndrome: In the context of vertebral fusions in mice and humans, different effects of BMP signaling have been reported: Recessively inherited loss-of-function mutations in BMP13, also named GDF6 (5), or its close relative GDF3 (6) cause Klippel-Feil-Syndrome (KFS), characterized by abnormal fusion of any two of the first seven cervical vertebrae in the neck, but usually restricted to vertebrae C1 – C3, all of which result from aberrant segmentation along the embryo’s developing axis during the first 3 to 8 weeks of gestation (7).