A gene probe is a specific segment of single-strand DNA that is complementary to a desired gene. For example, if the gene of interest contains the sequence AATGGCACA, then the probe will contain the complementary sequence TTACCGTGT. When added to the appropriate solution, the probe will match and then bind to the gene of interest. To help locate the probe during the procedure, scientists usually label it with a radioisotope or a fluorescent dye so that it can be seen and identified.