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To speed up the walk, genomic libraries containing very large cloned DNA molecules are optimal. To probe for the next clone in the walk by DNA hybridization, a short DNA fragment (labeled with a chemical or a radioisotope) from one end of the previously identified clone is purified: If a "right-handed" end is used, for example, the walk will go in the "rightward" direction, as shown in this example. Use of a small end fragment as a probe also reduces the probability that the probe will contain a repeated DNA sequence that would hybridize with many clones from different parts of the genome and thereby interrupt the walk.