This chromosome walk begins with a recombinant phage or BAC clone obtained from a library that contains large inserts representing an entire eukaryotic genome. In the example shown, the molecular marker 7q22 is used to probe a human genomic library. Only the insert DNAs are shown. The insert DNA selected by the probe is then used to isolate another recombinant phage or BAC containing a neighboring segment of eukaryotic DNA. This walk illustrates how to start at molecular landmark 7q22 and get to marker 7q31.1, which is on the other side of the CF gene.