A method for generating a nucleic acid molecule capable of binding to a target molecule, wherein the nucleic acid molecule is an aptamer or a spiegelmer, wherein the binding affinity of the nucleic acid molecule is increased relative to the binding affinity of a reference nucleic acid molecule to the target molecule, wherein the method comprises the following steps: a) providing the reference nucleic acid molecule, wherein the reference nucleic acid molecule is capable of binding to the target molecule and wherein the reference nucleic acid molecule comprises a sequence of nucleotides, wherein the sequence of nucleotides comprises n nucleotides; b) preparing a first level derivative of the reference nucleic acid molecule, wherein the first level derivative of the reference nucleic acid molecule differs from the reference nucleic acid molecule at one nucleotide position, wherein the first level derivative is prepared by replacing the ribonucleotide at the one nucleotide position by a 2'-deoxyribonucleotide in case the reference nucleic acid has a ribonucleotide at the nucleotide position and wherein the first level derivative is prepared by replacing the 2'-deoxyribonucleotide at the one nucleotide position by a ribonucleotide in case the reference nucleic acid has a 2'-deoxyribonucleotide at the nucleotide position and wherein the nucleotide position at which the replacement is made is the modified nucleotide position; c) repeating step b) for each nucleotide position of the reference nucleic acid molecule, thus preparing a group of first level derivatives of the reference nucleic acid molecule, wherein the group of first level derivatives of the reference nucleic acid molecule consists of n first level derivatives, wherein each of the first level derivatives of the reference nucleic acid molecule differs from the reference nucleic acid molecule by a single nucleotide replacement and wherein each of the first level derivatives of the reference nucleic acid molecule has a single modified nucleotide position which is different from the single modified nucleotide of all of the single modified nucleotide positions of the other first level derivatives of the group of first level derivatives of the reference nucleic acid molecule; d) determining the binding affinity to the target molecule for each of the n first level derivatives of the reference nucleic acid; and optionally e) identifying the/those first level derivative(s) of the reference nucleic acid molecule the binding affinity of which exceed a predetermined threshold value. a) providing the reference nucleic acid molecule, wherein the reference nucleic acid molecule is capable of binding to the target molecule and wherein the reference nucleic acid molecule comprises a sequence of nucleotides, wherein the sequence of nucleotides comprises n nucleotides; b) preparing a first level derivative of the reference nucleic acid molecule, wherein the first level derivative of the reference nucleic acid molecule differs from the reference nucleic acid molecule at one nucleotide position, wherein the first level derivative is prepared by replacing the ribonucleotide at the one nucleotide position by a 2'-deoxyribonucleotide in case the reference nucleic acid has a ribonucleotide at the nucleotide position and wherein the first level derivative is prepared by replacing the 2'-deoxyribonucleotide at the one nucleotide position by a ribonucleotide in case the reference nucleic acid has a 2'-deoxyribonucleotide at the nucleotide position and wherein the nucleotide position at which the replacement is made is the modified nucleotide position; c) repeating step b) for each nucleotide position of the reference nucleic acid molecule, thus preparing a group of first level derivatives of the reference nucleic acid molecule, wherein the group of first level derivatives of the reference nucleic acid molecule consists of n first level derivatives, wherein each of the first level derivatives of the reference nucleic acid molecule differs from the reference nucleic acid molecule by a single nucleotide replacement and wherein each of the first level derivatives of the reference nucleic acid molecule has a single modified nucleotide position which is different from the single modified nucleotide of all of the single modified nucleotide positions of the other first level derivatives of the group of first level derivatives of the reference nucleic acid molecule; d) determining the binding affinity to the target molecule for each of the n first level derivatives of the reference nucleic acid; and optionally e) identifying the/those first level derivative(s) of the reference nucleic acid molecule the binding affinity of which exceed a predetermined threshold value.