Cleidocranial dysostosis

Cleidocranial dysostosis, also called Cleidocranial dysplasia, is a hereditary congenital disorder due to haploinsufficiency caused by mutations in the CBFA1 gene also called Runx2, located on the short arm of chromosome 6.

It is usually autosomal dominant, but in some cases the cause is not known.

Presentation
Cleidocranial dysostosis is a general skeletal condition so named from the collarbone (cleido-) and cranium deformities which people with it often have. Common features are:
 * Partly or completely missing collarbones. If the collarbones are completely missing or reduced to small vestiges, this allows hypermobility of the shoulders including ability to touch the shoulders together in front of the chest.
 * A soft spot or larger soft area in the top of the head where the fontanelle failed to close, or the fontanelle closes late.
 * Bones and joints are underdeveloped. People are shorter and their frames are smaller than their siblings who do not have the condition.
 * The permanent teeth include supernumerary teeth. Unless these supernumeraries are reabsorbed before adolescence, they will crowd the adult teeth in what already may be an underdeveloped jaw. In that case, the supernumeraries will probably need to be removed to provide space for the adult teeth.
 * Permanent teeth not erupting
 * Bossing (bulging) of the forehead.
 * Hypertelorism

Notable cases
The comedian Emmett Furrow has no collarbones and uses the resulting extra shoulder mobility in comedy routines. (2nd message); see also YouTube video

At the rescue of Jessica McClure, Ron Short, a muscular man (a roofing contractor) who was born without collarbones because of cleidocranial dysostosis and so could collapse his shoulders to work in cramped corners, arrived at the site and offered to go down the shaft; they accepted his offer, but did not use it.