Actinomycin D is a cyclic polypeptide-containing antibiotic that binds to DNA and blocks transcription. It binds to pre-melted double-stranded DNA in transcription bubbles and once bound is slow to dissociate. DNA in transcriptionally hyperactive cancer cells efficiently and rapidly bind actinomycin D at low concentrations (~10 nM), which interferes with RNA synthesis and ultimately leads to cell death. Actinomycin D is widely used as an anticancer agent for treating a variety of tumors and along with its fluorescent derivative, 7-aminoactinomycin D (Cay-11397) has become a useful tool in biochemistry and molecular biology research as a marker for DNA.