5-DBCO-PEG 4-dUTP is recommended for two-step labeling of DNA/cDNA e.g. by PCR. It is enzymatically incorporated into DNA/cDNA as substitute for its natural counterpart dTTP. The result-ing DBCO-functionalized DNA/cDNA can subsequently be labeled via Cu(I)-free click chemistry that offers the choice: to introduce a Biotin group (via Azides of Biotin) for subsequent purification tasks; to introduce fluorescent group (via Azides of fluorescent dyes) for subsequent microscopic imaging; to crosslink the DNA to Azide-functionalized biomolecules e.g.proteins.