A single-stranded DNA or RNA fragment used in genetic engineering to search for a particular gene or other DNA sequence. The probe has a base sequence complementary to the target sequence and will thus attach to it by base pairing. By labelling the probe with a radioactive isotope or fluorescent label it can be identified on subsequent separation and purification. Probes of varying lengths, up to about 100 nucleotides, can be constructed in the laboratory. They are used in the Southern blotting technique to identify particular DNA fragments, for instance in conjunction with restriction mapping to diagnose gene abnormalities or to map certain sequences. See also dna microarray.