Abstract: A method for detecting allelic imbalance (AI) in sample nucleic acid, which method comprises providing multiple copies of a target nucleic acid region present in the sample, the target nucleic acid region comprising a marker of heterozygosity, separating the multiple copies into individual strands then allowing the individual strands to reanneal under conditions which permit the formation of homoduplexes and heteroduplexes, removing any heteroduplexes so formed, subjecting the remaining homoduplexes to the above steps of separation, reannealing and heteroduplex removal one or more times so that any difference in the initial ratio of allelic variants is amplified, and detecting the presence or absence of AI by reference to any difference in allele ratio so detected.