Abstract: Process for the preparation of a deletion fragment from a target nucleic acid, the target having a region of known nucleotide sequence identical to one terminus of the deletion fragment. In the process, the target nucleic acid is provided as a single stranded molecule. A terminus primer which is homologous to the region of known nucleotide sequence, and a set of deletion primers each of which has a fixed site region, a spacer region and a tail are also provided. The set of deletion primers contain a mixture of oligonucleotides which have homologous fixed site regions and homologous tails, and which collectively have all four bases at each nucleotide position in essentially all combinations thereof in their spacer regions. The single strands are treated with the terminus primer and the set of deletion primers to form a primer extension product wherein the target nucleic acid is used as a template.