After 30 min, the reaction was supplemented to a final volume of 25 μL with 5 units of Klenow fragment (3′→5′ exo-) DNA polymerase (NEB, USA) and 5 μM of biotin-14-dUTP, dATP, dGTP and dCTP and incubated at 37 °C for another 30 min to fill in the overhangs and biotinylate the fragmented DNA.