Blunting is the elimination of incompatible 3’ or 5’ overhangs for the promotion of blunt-end ligation. Several approaches may be used for DNA end blunting. Terminal unpaired nucleotides may be removed from DNA ends by using an enzyme with exonuclease activity, which hydrolyzes a terminal phosphodiester bond, thereby removing the overhang one base at a time. DNA fragments with 5’ overhangs may be blunted by filling in a recessed 3’ terminus with DNA polymerase in the presence of dNTPs. End removal or fill-in can be accomplished using a number of enzymes, including DNA Polymerase I Large (Klenow) Fragment (NEB #M0210), T4 DNA Polymerase (NEB #M0203) or Mung Bean Nuclease (NEB #M0250). Once blunted, DNA is universally compatible with other blunt-ended fragments and vectors.