Figure 3: (a) Six main types of flavonoid which have been tested in melanoma models and their major dietary sources. (b) Structure comparison of luteolin, apigenin, and quercetin. The different side groups are circled in luteolin structure. The boxed side groups in apigenin show a typical structure that is able to bind metal ions. Comparing these three popular compounds which sometimes show opposite effects on melanogenesis may provide some hints on how each side group functions biologically.