Chromosomes can be specifically stained in
metaphase spreads and interphase nuclei by in situ hybridization
with entire chromosome-specific DNA libraries. Unlabeled
human genomic DNA is used to inhibit the hybridization of
sequences in the library that bind to multiple chromosomes.
The target chromosome can be made at least 20 times brighter
per unit length than the others. Trisomy 21 and translocations
involving chromosome 4 can be detected in metaphase spreads
and interphase nuclei by using this technique.