In benzo-homologated purines (xA and xG), the benzene ring is bound to the nitrogenous base through nitrogen-carbon (N-C) bonds. Benzo-homologated pyrimidines are formed through carbon-carbon (C-C) bonds between the base and the benzene. Thus far, x-nucleobases have been added to strands of DNA using phosphoramidite derivatives, as traditional polymerases have been unsuccessful in synthesizing strands of xDNA. X-nucleotides are poor candidates as substrates for B-DNA polymerases as their size interferes with binding at the catalytic domain. Attempts at using template-independent enzymes have been successful as they have a reduced geometric constraint for substrates. Terminal deoxynucleotidyl transferase (TdT) has been used previously to synthesize strands of bases which have been bound to fluorophores. Using TdT, up to 30 monomers can be combined to form a double-helix of xDNA, however this oligomeric xDNA appears to inhibit its own extension beyond this length due to the overwhelming hydrogen bonding. In order to minimize inhibition, xDNA can be hybridized into a regular helix.