The photochemically reactive sites in psoralens are the alkene-like carbon-carbon double bonds in the furan ring (the five-member ring) and the pyrone ring (the six-member ring). When appropriately intercalated adjacent to a pyrimidine base, a four-center photocycloaddition reaction can lead to the formation of either of two cyclobutyl-type monoadducts. Ordinarily, furan-side monoadducts form in a higher proportion. The furan monoadduct can absorb a second UVA photon leading to a second four-center photocycloaddition at the pyrone end of the molecule and hence the formation of a diadduct or cross-link. Pyrone monoadducts do not absorb in the UVA range and hence cannot form cross-links with further UVA irradiation.