Genomic DNA is shown in blue. Selected clones from a library of cloned genomic DNA fragments are shown in red. The initial probe, probe a, is specific to gene A or exon A and allows identification of clones 1 and 2. A new probe, probe b, is prepared from one end of clone 2 and used to isolate new clones 3 and 4 from the genomic library. Probe c, prepared from clone 4 is used to idenitify clone 5, etc. The orientation of the clones is determined by restriction mapping of the clones. Clone 6 contains the desired gene B or exon B.