Photoreactive agents, on exposure to ultraviolet light, become active and bind non specifically with neighbouring molecules. These reagents are suitable for labelling molecules that do not contain easily reactable functional groups. Photoreactive reagent HOOK™ Psoralen-PEO-Biotin reacts with and labels nucleic acids and protein molecules. When reacted with nucleic acids, it crosslinks with pyrimidine bases. Crosslinking does not interfere with hybridisation applications.