The polymerase chain reaction (PCR) is used for selective amplification of DNA fragments from both prokaryotes and eukaryotes (1-3). The only requirement for amplification is that the sequence of the extremities of the DNA fragment to be amplified be known (4). This places a limitation on the use of PCR in the amplification of adjacent unknown regions. We have developed a method that allows the amplification of double-stranded DNA even when the sequence information is available at one end only (5). This method, the single specific primer-PCR (SSP-PCR), permits amplification of genes for which only a partial sequence information is available, and allows unidirectional genome walking from known into unknown regions of the chromosome.