Two cases of Down syndrome with unusual de novo translocation.

Two children with the clinical features of Down syndrome were found to have several unusual cell lines. In both cases the same reverse tandem translocation between two 21 chromosomes was present in one line. This may be an unstable rearrangement. In addition, the findings offer some support for current efforts to localize the portion of chromosome 21 responsible for clinical features of Down syndrome to band 21q22. Acridine orange R banding was found to be especially useful in the identification of the break points on the translocations. The origin of the abnormality was found to be paternal in one case and was indeterminate in the second.