Partial trisomy of chromosome 21 without the Down syndrome phenotype

What's Already Known About This Topic? Down syndrome (DS) is a common disorder associated with several complex clinical phenotypes. It is unclear as to whether particular loci on chromosome 21 are sufficient to cause DS and its associated features. What's Already Known About This Topic? We reported two prenatally detected cases of partial trisomy 21, and both were inherited from their healthy mothers and without DS phenotypes. These two segments of partial trisomy 21 may suggest copy number variants or minor phenotypes with variable penetrance in the regions of chromosome 21. Oligonucleotide-based array comparative genomic hybridization can be helpful in mapping high-resolution segments of chromosome 21, and narrowing down the DS critical region.