DNA polymerases (EC 2.7.7.7) can be classified, on the basis of sequence similarity [1], into at least four different groups: A, B, C and X. DNA polymerases that belong to family X are listed below [2]:

• Vertebrate polymerase β, involved in DNA repair.
• Yeast polymerase IV (POL4) [3], an enzyme with similar characteristics to that of the mammalian polymerase β.
• Terminal deoxynucleotidyltransferase (TdT) (EC 2.7.7.31). TdT catalyzes the elongation of polydeoxynucleotide chains by terminal addition. One of the functions of this enzyme is the addition of nucleotides at the junction of rearranged Ig heavy chain and T cell receptor gene segments during the maturation of B and T cells.
• African Swine Fever virus protein O174L [4].
• Fission yeast hypothetical protein SpAC2F7.06c.

These enzymes are small (about 40 Kd) compared with other polymerases and their reaction mechanism operates via a distributive mode, i.e. they dissociate from the template-primer after addition of each nucleotide.

As a signature pattern for this family of DNA polymerases, we selected a highly conserved region that contains a conserved arginine and two conserved aspartic acid residues. The latter together with the arginine have been shown [5] to be involved in primer binding in polymerase β.