Fimers are primers with proprietary chemical modifications that inhibit unwanted reactions during cycle sequencing. They are prepared from oligonucleotides possessing one or more SUC precursors. By carefully selecting the position of precursor incorporation on the oligo we can select the optimal Fimers to inhibit primer-dimer extension and non-specific PCR for sequencing reactions. It has been shown that Fimers decrease the T_m of the duplex formed with its unmodified complementary strand and improve the annealing specificity, thus contributing to the suppression of non-specific PCR. Furthermore, no primer-dimer extension products were detected by placing the modified nucleotide at the -7 position from the primer 3′-end having complementary bases to itself, and high quality sequence from the template DNA was obtained.