Psoralens and their derivatives (such as 8-TMP and 4,5’,8-TMP) are well known to have unique crosslinking features to DNA. Psoralen TMP Maleimide is a thiol-reactive TMP derivative. It is an excellent building block for preparing TMP-labeled oligos from the readily available thiol-modified oligos. The TMP-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 TMP Maleimide may also be used for preparing site-specific DNA/RNA probes via the conjugations with thiol-containing biomolecules such as antibodies.