With the cDNA map, the positions of exons, relative to bands or specific parts of the chromosome, is shown. A mRNA molecule us used as a template for the synthesis of cDNA in the laboratory. The rules for base-pairing still follow and are applied (i.e. adenine and thymine still pair up) and this cDNA is then mapped to genomic regions.

  

    This is a picture of the genes that make up DNA which makes up a chromosome.  This picture shows how guanine and cytosine still pair up and thymine and adenine pair up.

    CDNA's are thought to identify the most biological and medically significant parts of the genome. This is because they only represent exons. Using a cDNA map, the chromosomal location of genes whose functions are unknown can be found.

   When the approximate location of a disease gene has been mapped by genetic linkage techniques, a cDNA map can suggest a set of genes to test which may posses a code for the disease being looked for.