Screening recombinants.
<P>
   Only  a  cell  that  took  up  a plasmid which has the amp^R gene will
   reproduce and form a colony.
<P>
   Cells with non-recombinant plasmids have a functional lacZ enzyme that
   allows them to digest sugar and form blue colonies.
<P>
   Some  of  the recombinant clones have a disrupted lacZ gene; they will
   form white colonies.
<P>
   These  white  colonies  can be analyzed to identify those carrying the
   gene of interest.