Random truncation and amplification of nucleic acid

A method for producing a library of mutagenized polynucleotide from a target sequence comprising:

• (a) taking a sample comprising
• (i) a target sequence including a section to be mutagenized,
• (ii) a library of first primers where the first primers include a first fixed sequence and a first random sequence 3′ to the first fixed sequence, the first fixed sequence being insufficiently homologous to the target sequence to anneal to the target sequence under primer extension amplification conditions and the first random sequence varying within the library of first primers and having at least 5 nucleotides in length, and
• (iii) a library of second primers where the second primer include a second fixed sequence that differs from the first fixed sequence, and a second random sequence 3′ to the second fixed sequence, the second random sequence varying within the library of second primers and having at least 5 nucleotides in length;
• and
• (b) performing multiple cycles of primer extension amplification on the sample using a polymerase where primer extension is performed under conditions suitable for the first random sequence of a member of the library of first primers and the second random sequence of a member of the library of second primers to separately anneal to a portion of the target sequence or amplification products thereof and be extended by the polymerase;
• wherein a library of mutagenized polynucleotides are produced as amplification products of the multiple amplification cycles.