A novel DNA cloning vector, lambda MT-5 has been constructed by recombinant DNA techniques. This vector is used to generate eukaryotic genomic libraries that can be screened with the amber suppressible mini-plasmid piVX. This enables a positive selection of recombinants without a background of the parent vector. In an analogous fashion we have developed techniques for isolating unique DNA recombinants in human mos onc gene between human genomic DNA and the mos region of Moloney sarcoma virus. We have characterized these recombinants by specific methods demonstrating that such recombinants contain both human and MSV mos genetic sequences. These recombinants will be tested for biological transforming activity on NIH 3T3 cells.