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.