In molecular biology, a hybridization probe is a fragment of DNA of variable length (usually 100-1000 bases long), which is used to detect in DNA or RNA samples the presence of nucleotide sequences that are complementary to the sequence in the probe. The hybridization probe is labeled radioactively (commonly with 32P) or with immunological markers, such digoxigenin. The labeled probe is then denatured (by heating) into single DNA strands and hybridized to target DNA (Southern blotting) or RNA (northern blotting) immobilized on a membrane or in situ. DNA sequences or RNA transcripts that have moderate to high (depending on the stringency in the hybridization) sequence similarity to the probe, are detected by visualizing the hybridized probe via autoradiography or other imaging techniques. Hybridization probes used in DNA microarrays refer to DNA covalently attached to an inert surface, such as coated glass slides or gene chips, and to which a mobile DNA target is hybridized.