Abstract: A method of DNA library screening includes homologous recombination in E. coli utilizing lambda phage recombination functions. Inserting a positive selection marker such as antibiotic resistance into the target sequence by homologous recombination facilitates isolation of target sequences and requires only about 58-100 base pairs of total homology, thus allowing the use of synthetic fragments of DNA for targeting. DNA vector is designed for genomic library construction that features a novel genetic selection for inserts, automatic subcloning of isolated genomic clones and the presence of a negative selection marker adjacent to the genomic inserts to facilitate later gene targeting.