The authors have developed a method called primer-extension preamplification (PEP) to amplify a large fraction of the DNA sequences present in a single haploid cell. This method uses repeated primer extensions with a mixture of 15-base random oligonucleotides and Taq DNA polymerase. The study tested 12 genetic loci and found that at least 78% of the genomic sequences in a single human haploid cell can be amplified to a minimum of 30 copies. This method has significant implications for multipoint mapping by sperm or oocyte typing, genetic disease diagnosis, forensics, and the analysis of ancient DNA samples. The PEP method does not require extensive sample manipulation and can be used to confirm genotypes in single-cell studies, allowing for more accurate and reliable genetic analysis.The authors have developed a method called primer-extension preamplification (PEP) to amplify a large fraction of the DNA sequences present in a single haploid cell. This method uses repeated primer extensions with a mixture of 15-base random oligonucleotides and Taq DNA polymerase. The study tested 12 genetic loci and found that at least 78% of the genomic sequences in a single human haploid cell can be amplified to a minimum of 30 copies. This method has significant implications for multipoint mapping by sperm or oocyte typing, genetic disease diagnosis, forensics, and the analysis of ancient DNA samples. The PEP method does not require extensive sample manipulation and can be used to confirm genotypes in single-cell studies, allowing for more accurate and reliable genetic analysis.