DBCO-18-dUTP, also called 5-DBCO-PEG4-dUTP, is used 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 resulted DBCO-functionalized DNA/cDNA can subsequently be labeled via Cu-free click chemistry that is also known as strain-promoted azide-alkyne cycloaddition (SPAAC) with a desired tag such as biotin, a fluorophore or a biopolymer, e.g., an enzyme or antibody.