The term FISH refers to the common practice of fluorescence in situ hybridisation conducted on preparations of extended chromatin fibers. When mapping DNA fragments of interest by conducting FISH investigations on chromosomes, signals within a distance of several million base pairs are indistinguishable from each other because of the highly supercoiled structure of DNA strands in the metaphase chromosomes. The resolution of signals improves if the chromosomes are used before they progress to full condensation. What can we do when we want to map more adjacent DNA segments? The characterisation of entire genome DNA sequences will resolve the problem of creating a map in scale of one base pair, but is extremely time-consuming.