DNA shuffling (also called molecular breeding) generates variation by random fragmentation of a cloned gene followed by reassembly of the fragments in a self-priming polymerase reaction. The result is a recombination of overlapping fragments that have different mutations or come from different, naturally occurring homologous genes. In the August Nature Genetics Soong et al apply this technique to a pool of six different murine leukemia virus envelope sequences to derive a new virus that can, unlike its parents, infect Chinese Hamster Ovary (CHOK1) cells (Nat. Gen. 2000, 25:436-439). Similar selections on clinically relevant cell types may yield improved vectors for gene therapy.