Cutting edge researchers demonstrate step-by-step how oligonucleotide primers may be successfully used to detect and amplify or extend complimentary sequences in situ. Through these procedures-often invented by the authors-the door is opened to rapid identification and characterization of chromosomal DNA sequences, viral genomes, and rare messenger RNAs in cells-at hitherto unmatched degrees of sensitivity and specificity. Their innovative techniques-suitable for both novice and experienced researchers-have rapidly become indispensable for many clinical diagnostic procedures, whether in quantification of chromosomes in the identification of aneuploidy for prenatal diagnosis, or in the identification of viral infection in the early stages, or of rare messenger RNAs present in cells.