18q− and 18q+ mosaicism in a mentally retarded boy

Abstract
A mentally retarded boy was found to have an unusual chromosomal mosaicism [46,XY, del(18) (q22)/46,XY,iso psu dic(18)(q23)]. The clinical manifestations are compatible with the 18q− syndrome. The chromosome alteration was denned by high resolution banding and fluorescence in situ hybridization (FISH). A mechanism to explain the origin of the two cell lines is presented and discussed.