Here we describe template directed enzymatic synthesis of unique primers, avoiding the chemical synthesis step in primer walking. We have termed this conceptually new technique DENS (differential extension with nucleotide subsets). DENS works by selectively extending a short primer, making it a long one at the intended site only. The procedure starts with a limited initial extension of the primer (at 20-30 degrees C) in the presence of only two out of the four possible dNTPs. The primer is extended by 6-9 bases or longer at the intended priming site, which is deliberately selected, (as is the two-dNTP set), to maximize the extension length. The subsequent termination reaction at 60-65 degrees C then accepts the extended primer at the intended site, but not at alternative sites, where the initial extension (if any) is generally much shorter. DENS allows the use of primers as long as 8mers (degenerate in two positions) which prime much more strongly than modular primers involving 5-7mers and which (unlike the latter) can be used with thermostable polymerases, thus allowing cycle-sequencing with dye-terminators compatible with Taq DNA polymerase, as well as making double-stranded DNA sequencing more robust.