Incidence of low-fluorescence alpha satellite region on chromosome 21 escaping detection of aneuploidy at interphase by FISH.

Aneuploidy detection for chromosome 21 by fluorescence in situ hybridization (FISH) to interphase nuclei using a probe specific for the alphoid DNA sequences D21Z1/D13Z1 should be avoided. An extreme heteromorphism, resulting in misdiagnosis if interphase FISH is the only test employed, may be far more frequent (4/101) than expected.