A technique for detection and amplification of target DNA sequences. Two oligodeoxynucleotides are synthesised which between them are complementary to the entire target sequence, one to the 5'-side and one to the 3'-side. If the target sequence is present in the DNA sample under examination, the oligonucleotides will bind to it at moderate temperature with their ends abutting in the center, and a heat-stable ligase will join them into a complete polynucleotide. No ligation will occur if the target sequence is absent or if the match between synthetic oligonucleotides and target sequence is imperfect in the region where they abut.

At an elevated temperature the new polynucleotide dissociates from the original DNA template, and upon cooling to moderate temperature it and the original DNA serve as templates for a second cycle of hybridization, ligation and thermal dissociation. At each cycle there is a doubling of the number of new complete polynucleotides, so after 20 cycles there can be a 10⁶-fold amplification. The half-sequences may be constructed with markers on the non-ligating ends to assist isolation (e.g. biotin) or detection (e.g. a fluorophore).