composition on the enzymatic incorporation into DNA was systematically investigated using two different DNA polymerases. Deep VentR exo− from the B-polymerase family accepted most nucleotide analogues as substrates, while Taq from the A-family was slightly less proficient. Both polymerases had difficulties incorporating 5-(3-amino-prop-1-ynyl)-2′-deoxyuridine triphosphate. A molecular model of the active