The present invention relates to the detection of a target nucleic acid sequence by a PTOCE (PTO Cleavage and Extension) assay. The present invention detects a target nucleic acid sequence in which the PTO (Probing and Tagging Oligonucleotide) hybridized with the target nucleic acid sequence is cleaved to release a fragment and the fragment is hybridized with the CTO (Capturing and Templating Oligonucleotide) to form an extended duplex, followed by detecting the presence of the extended duplex. The extended duplex provides signals (generation, increase, extinguishment or decrease of signals) from labels indicating the presence of the extended duplex and has adjustable Tm value, which are well adoptable for detection of the presence of the target nucleic acid sequence.