A method for detecting a target nucleic acid sequence from a DNA or a mixture of nucleic acids by a PCE-SC (PTO Cleavage and Extension-Dependent Signaling Oligonucleotide Cleavage) assay, comprising: (a) hybridizing the target nucleic acid sequence with an upstream oligonucleotide and a PTO (Probing and Tagging Oligonucleotide); wherein the upstream oligonucleotide comprises a hybridizing nucleotide sequence complementary to the target nucleic acid sequence; the PTO comprises (i) a 3'-targeting portion comprising a hybridizing nucleotide sequence complementary to the target nucleic acid sequence and (ii) a 5'-tagging portion comprising a nucleotide sequence non-complementary to the target nucleic acid sequence; wherein the 3'-targeting portion is hybridized with the target nucleic add sequence and the 5'-tagging portion is not hybridized with the target nucleic acid sequence; the upstream oligonucleotide is located upstream of the PTO; (b) contacting the resultant of the step (a) to an enzyme having a 5'-nuclease activity under conditions for cleavage of the PTO; wherein the upstream oligonucleotide or its extended strand induces cleavage of the PTO by the enzyme having a 5'-nuclease activity such that the cleavage releases a fragment comprising the 5'-tagging portion or a part of the 5'-tagging portion of the PTO; (c) hybridizing the fragment released from the PTO with a CTO (Capturing and Templating Oligonucleotide); wherein the CTO comprises in a 3' to 5' direction (i) a capturing portion comprising a nucleotide sequence complementary to the 5'-tagging portion or a part of the 5'-tagging portion of the PTO and (ii) a template portion comprising a nucleotide sequence non-complementary to the 5'-tagging portion and the 3'-targeting portion of the PTO; wherein the fragment released from the PTO is hybridized with the capturing portion of the CTO; (d) performing an extension reaction using the resultant of the step (c) and a template-dependent nucleic acid polymerase, wherein the fragment hybridized with the capturing portion of the CTO is extended to form an extended strand comprising an extended sequence complementary to the templating portion of the CTO, thereby forming an extended duplex; (e) hybridizing the extended strand with SO (Signaling Oligonucleotide) to form an extended strand/SO hybrid; wherein the SO comprises a hybridizing nucleotide sequence complementary to the extended strand and at least one label; (f) cleaving the SO of the extended strand/SO hybrid using a nucleolytic enzyme to generate a cleaved fragment of the SO; and (g) detecting the occurrence of the cleavage reaction in the step (f); wherein the detection is performed by measuring a signal provided from the label linked to the SO, whereby the occurrence of the cleavage reaction of the SO of the extended strand/SO hybrid indicates the presence of the target nucleic acid sequence. (a) hybridizing the target nucleic acid sequence with an upstream oligonucleotide and a PTO (Probing and Tagging Oligonucleotide); wherein the upstream oligonucleotide comprises a hybridizing nucleotide sequence complementary to the target nucleic acid sequence; the PTO comprises (i) a 3'-targeting portion comprising a hybridizing nucleotide sequence complementary to the target nucleic acid sequence and (ii) a 5'-tagging portion comprising a nucleotide sequence non-complementary to the target nucleic acid sequence; wherein the 3'-targeting portion is hybridized with the target nucleic add sequence and the 5'-tagging portion is not hybridized with the target nucleic acid sequence; the upstream oligonucleotide is located upstream of the PTO; (b) contacting the resultant of the step (a) to an enzyme having a 5'-nuclease activity under conditions for cleavage of the PTO; wherein the upstream oligonucleotide or its extended strand induces cleavage of the PTO by the enzyme having a 5'-nuclease activity such that the cleavage releases a fragment comprising the 5'-tagging portion or a part of the 5'-tagging portion of the PTO; (c) hybridizing the fragment released from the PTO with a CTO (Capturing and Templating Oligonucleotide); wherein the CTO comprises in a 3' to 5' direction (i) a capturing portion comprising a nucleotide sequence complementary to the 5'-tagging portion or a part of the 5'-tagging portion of the PTO and (ii) a template portion comprising a nucleotide sequence non-complementary to the 5'-tagging portion and the 3'-targeting portion of the PTO; wherein the fragment released from the PTO is hybridized with the capturing portion of the CTO; (d) performing an extension reaction using the resultant of the step (c) and a template-dependent nucleic acid polymerase, wherein the fragment hybridized with the capturing portion of the CTO is extended to form an extended strand comprising an extended sequence complementary to the templating portion of the CTO, thereby forming an extended duplex; (e) hybridizing the extended strand with SO (Signaling Oligonucleotide) to form an extended strand/SO hybrid; wherein the SO comprises a hybridizing nucleotide sequence complementary to the extended strand and at least one label; (f) cleaving the SO of the extended strand/SO hybrid using a nucleolytic enzyme to generate a cleaved fragment of the SO; and (g) detecting the occurrence of the cleavage reaction in the step (f); wherein the detection is performed by measuring a signal provided from the label linked to the SO, whereby the occurrence of the cleavage reaction of the SO of the extended strand/SO hybrid indicates the presence of the target nucleic acid sequence.