Note that the vectorettes contain complementary sequences at their ends but unrelated sequences within the bubble. A restriction fragment containing a known sequence A flanked by uncharacterized regions X and Y is ligated to a vectorette linker containing a suitable overhang at one end. PCR amplification of the uncharacterized sequence X is then possible using a primer specific for the known DNA sequence (A) and a primer specific for one of the strands of the bubble in the vectorette linker (B). The vectorette linker primer B cannot prime DNA synthesis initially as there is no sequence to which it can bind: it is identical, not complementary in sequence to one strand of the bubble, and unrelated in sequence to the other. However, primer A initiates synthesis of a complementary DNA strand which will contain a sequence complementary to one of the unique sequences within the bubble linker. As a result, primer B can bind to this newly synthesized strand and initiate new strand synthesis to start a PCR reaction. The flanking sequence Y can similarly be isolated in another reaction using a suitable A-specific primer and a bubble-specific primer derived from the strand opposite to that used for making primer B.