Ethidium bromide (EtBr, homidium bromide)

• fluorescent toxicant (toxic man-made chemical) widely used as a molecular probe because it interacts with DNA and fluoresces with increasing intensity when it binds to DNA
• used as a stain to detect DNA as part of certain molecular biological techniques including microscopy and gel electrophoresis
• is an intercalating agent, which means that it is capable of inserting itself between the paired stands of DNA, deforming it and potentially messing up important cellular processes like DNA replication and transcription
• is strongly mutagenic, as it causes frameshift mutations in several strains of Salmonella typhimurium, a species of yeast used to assay mutagenicity (see: the Ames test), when it is metabolically activated by CYP P450-dependent mixed-function oxidases (a class of liver enzymes that oxidize chemicals)
• this suggests that it may be carcinogenic and teratogenic
  
• also used to treat animal trypanosomiasis (parasites) since they bring the reaper down on these little nasties nice 'n well
  

Tettey JN et al. The effect of inducing agents on the metabolism of ethidium bromide by isolated rat hepatocytes. Chem Biol Interact. 1999 Nov 30;123(2):105-15.