this gene have a 78% chance of having aural atresia. Critical regions – Recent research has narrowed the critical regions for four features of the distal 18q- phenotype down to a small segment of distal 18q, although the precise genes responsible for those features remain to be identified. The table below shows the established critical regions for four features of distal 18q-, as well as the penetrance for each of those features. The penetrance figure represents the likelihood a person would have the feature given the critical region is deleted. Haplolethal regions - Two regions on chromosome 18 have never been found to be deleted. They are located between the centromere and 22,826,284 bp (18q11.2) and between 43,832,732 and 45,297,446 bp (18q21.1). The genes in these regions are thought to be lethal when deleted. See also Proximal 18q- References External links Autosomal monosomies and deletions Syndromes affecting the nervous system