Psoralens and their derivatives (such as 8-MOP and 4,5’8-TMP) are well known to have unique crosslinking features to DNA. However, psoralen monomers do not have sequence-specific crosslinking ability with a target DNA. Nakao et al found that 5-MOP more effectively crosslinks DNA compared to the well-known 8-MOP (https://doi.org/10.1002/cbic.202200789). Psoralen MOP Succinimidyl Ester is a 5-MOP derivative. It is an excellent building block for preparing MOP-labeled oligos from the readily available amino-modified oligos. The 5-MOP-conjugated oligonucleotides can be used for sequence-specific crosslinking with a target DNA, thus enabling the application of psoralen-conjugated molecules in gene transcription inhibition, gene knockout, and other genomic applications. Psoralen MOP Succinimidyl Ester may also be used for preparing site-specific DNA/RNA probes via the conjugations with amino-containing biomolecules such as antibodies.