SuperCos 1 is a novel, 7.9-kb cosmid vector that contains bacteriophage promoter sequences flanking a unique cloning site (see Figure 1). This structure allows rapid synthesis of "walking" probes specific for the extreme ends of insert DNA. The SuperCos 1 vector is also engineered to contain genes for the amplification and expression of cosmid clones in eukaryotic cells. In addition, most genomic inserts can be excised as a single large restriction fragment using the Not I restriction site that flanks the SuperCos 1 polylinker.