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A cervical rib is a supernumerary (or extra) rib which arises from the seventh cervical vertebra. It is a congenital abnormality located above the normal first rib. A cervical rib is present in only about 1 in 200 (0.5%) of people; in even rarer cases, an individual may have not one but two cervical ribs.Associated conditionsThe presence of a cervical rib can cause a form of thoracic outlet syndrome due to compression of the lower trunk of the brachial plexus or subclavian artery. These structures are entrapped between the cervical rib and scalenus muscle.Compression of the brachial plexus may be identified by weakness of the muscles around the muscles in the hand, near the base of the thumb. Compression of the subclavian artery is often diagnosed by finding a positive Adson's sign on examination, where the radial pulse in the arm is lost during abduction and external rotation of the shoulder.Children born with cervical ribs develop early childhood cancer at a rate 125 times higher than the general population. The Hox genes that control the development of cervical vertebrae are believed to play a role in suppressing cancer.