A technique with which an unknown region of a Chromosome can be explored. It is Generally used to isolate a locus of interest for which no probe is available but that is known to be linked to A Gene which has been identified and cloned. A fragment containing a known Gene is selected and used as a probe to identify other overlapping fragments which contain the same Gene. The Nucleotide Sequences of these fragments can then be characterized. This process continues for the length of the Chromosome.