text: Fluorescence in situ hybridization (FISH) is a laboratory method used to detect and locate a DNA sequence, often on a particular chromosome.
  In the 1960s, researchers Joseph Gall and Mary Lou Pardue found that molecular hybridization could be used to identify the position of DNA sequences in situ (i.e., in their natural positions within a chromosome). In 1969, the two scientists published a paper demonstrating that radioactive copies of a ribosomal DNA sequence could be used to detect complementary DNA sequences in the nucleus of a frog egg. Since those original observations, many refinements have increased the versatility and sensitivity of the procedure to the extent that in situ hybridization is now considered an essential tool in cytogenetics.