DNA-binding molecules, including intercalators and groove binders, are usually positively charged. The main modes of reversible ligand binding to double-stranded DNA (dsDNA) are (i) intercalation between DNA base pairs, (ii) interactions with the DNA minor groove, (iii) interactions with the DNA major groove, and (iv) electrostatic interactions with the anionic sugar-phosphate backbone of DNA. Classical intercalators have planar fused aromatic ring systems, which stack between base pairs in the intercalation complex. Groove binding compounds have unfused aromatic ring systems with the conformation needed to fit into the helical curvature of DNA grooves.