– Aminomethyltrioxsalen hydrochloride, a Psoralen derivative, penetrates intact cells and, upon ultraviolet light irradiation, creates interstrand crosslinks between pyrimidine residues in nucleic acid helices. As a bifunctional nucleic acid intercalating agent, it serves as a probe for identifying double-stranded regions in heterogeneous nuclear RNA (hnRNA) to analyze its secondary structure.