Ethidium bromide is likely the most well-known dye used for visualizing DNA. It can be used in the gel mixture, the electrophoresis buffer, or to stain the gel after it is run. Molecules of the dye adhere to DNA strands and fluoresce under UV light, showing you exactly where the bands are within the gel. Despite its advantage, the downside is that ethidium bromide is a potential carcinogen, so it must be handled with great care.