Enzymatic addition of fluorescein- or biotin-riboUTP to oligonucleotides results in primers suitable for DNA sequencing and PCR.

An enzymatic, post-synthetic, non-isotopic modification of oligonucleotides giving primers that are substrates for chain elongation by DNA polymerases is described. It is shown that terminal deoxynucleotidyl transferase incorporates preferentially and almost exclusively a single fluorescein- or biotin-riboUTP at the 3' terminus of oligonucleotides. This one-step procedure, using readily available materials, permits an economical enzymatic labeling of oligonucleotides designed for fluorescence-based or solid-state DNA sequencing or PCR and offers a convenient alternative to chemical modification.