In order to enhance a collection of modified deoxynucleoside triphosphates useful for in vitro selection or SELEX (systematic evolution of ligands by exponential enrichment) techniques, we designed and synthesized modified analogues of 2'-deoxyuridine triphosphate and 2'-deoxycytidine triphosphate bearing a flexible and hydrophilic 7-amino-2,5-dioxaheptyl linker at a C5 position. Both analogues were found to be substrates for thermostable DNA polymerases which belong to an evolutional family B during PCR.