Psoralen belongs to a group of organic chemicals known as furocoumarins. They cannot achieve therapeutic effects on their own; they must be used in combination with UV light therapy to treat skin conditions. Psoralen compounds can intercalate with DNA and RNA molecules. Upon exposure to UVA light at 365 nm, psoralens undergo a chemical reaction that results in cross-linking between pyrimidine bases in DNA. This cross-linking is stable and disrupts DNA replication, causing cells to arrest in the cell cycle.