Abstract: A method is described for the detection of a rare deletion mutant allele in a population of wild-type individuals. DNA samples from the individuals are subjected to a modified form of polymerase chain reaction (PCR) which favors the replication of truncated DNA strands over the synthesis of wild-type full length strands. The preferred way to bias the process toward the synthesis of truncated DNA strands is by limiting the extension step in the PCR reaction protocol. This process permits the convenient detection of a deletion mutant allele from a population of 1000 wild type individuals with the full length allele.