Furanocoumarin derivatives, tricyclic ring systems in which the furan ring is linearly linked to the coumarin. The psoralens, as linear 7,6 - furanocoumarins are photosensitizing. PUVA therapy with 8-methoxypsoralen(ammoidine) is based on their photosensitizing capacity (cf. PUVA). Psoralens occur naturally in many Rutaceae, Apiaceae and Fabaeceae.