Abstract: A method for detecting and locating mutations in DNA involves forming heteroduplex molecules by hybridizing single strands derived from a sample of target DNA under test with single strands derived from a sample of non-mutant reference nucleic acid so that any mutation causing an alteration in one or more nucleotide bases in the target DNA produces a base pair mismatch in the corresponding heteroduplex molecule. The nucleic acid mixture is then reacted with a mismatch-binding protein such as the mismatch repair enzyme Mut"S" which recognizes and binds to any such resultant mismatch site. Subsequent treatment with an exonuclease having unidirectional activity degrades duplex molecules free of mismatches but mismatch-containing heteroduplex molecules are protected by the mismatch-binding protein bound to the mismatch sited therein and this limits the extent of the exonuclease degradation. The degradation products are then analyzed, e.g.