Genomic  libraries  are  screened  by  hybridization with  a  DNA probe which is complementary  to component of the nucleotide  sequence  of the desired gene. The problem should be a DNA restriction fragment or perhaps element of a cDNA clone.  Exchange  approach  is  possible  if  some  of  the  protein  sequence  for  the preferred  gene is known.  Whereas using the genetic code one can then reduced DNA sequence of this category of the gene and synthesize an oligonucleotide with this sequence to act as the DNA probe.