The technique for labeling chromosomes by annealing an oligonucleotide DNA primer to the denatured DNA of chromosome preparations on glass slides and extending it enzymaticallyin situwith the incorporation of labeled nucleotides was first described by Koch et al. in 1989 (1 ). Since then, the technique has been greatly improved in reliability, sensitivity, and resolution, and now provides a viable, rapid alternative to conventional fluorescencein situhybridization (FISH) for many investigations, particularly the identification of chromosome aneuploidy in metastatic tissues and antenatal diagnosis and the analysis of the human chromosome complement of somatic hybrid cell lines (2 –6 ).
