A method for detection of single base substitutions in DNA fragments. The PCR-amplified putatively mutated DNA fragment is mixed with homologous normal DNA fragments; any mutated sequence is much more dilute than the normal sequence and will therefore be less likely to re-anneal. They will appear on polyacrylamide-gel electrophoresis as band mobility shifts, especially under mildly denaturing conditions, and will indicate the presence of heteroduplexes.