Cervical C2/C3 vertebral fusion (Human Phenotype Ontology id HP_0004602) is fusion of cervical vertebrae at C2 and C3, caused by a failure in the normal segmentation or division of the cervical vertebrae during the early weeks of fetal development, leading to a short neck with a low hairline at the back of the head, and restricted mobility of the upper spine. Also known as: Fusion of cervical vertebrae c2-3, Klippel-Feil anomaly, Klippel-Feil syndrome.