Probe -- General term for a piece of DNA or RNA corresponding to a gene or sequence of interest, that has been labelled either radioactively, or with some other detectable molecule, such as biotin, digoxygenin or fluorescein. As stretches of DNA or RNA with complementary sequences will (hybridise), a probe will label viral plaques, bacterial colonies or bands on a gel that contain the gene of interest. See also Northern blots, Southern blots.