Familial 4.3 Mb duplication of 21q22 sheds new light on the Down syndrome critical region.

A 4.3 Mb duplication of chromosome 21 bands q22.13-q22.2 was diagnosed by interphase fluorescent in situ hybridisation (FISH) in a 31 week gestational age baby with cystic hygroma and hydrops; the duplication was later found in the mother and in her 8-year-old daughter. All had the facial gestalt of Down syndrome (DS). This is the smallest accurately defined duplication of chromosome 21 reported with a DS phenotype. The duplication encompasses the gene DYRK1 but not DSCR1 or DSCAM. Previous karyotype analysis and telomere screening of the mother, and karyotype analysis and metaphase FISH of a chorionic villus sample, had all failed to reveal the duplication. The findings in this family add to the identification and delineation of a "critical region" for the DS phenotype on chromosome 21.