Genes, which are housed in the DNA of the cell's nucleus, contain codes that essentially are recipes for tens of thousands of proteins. The code-containing regions of the gene (exons), however, are often separated by much noncoding DNA (introns). A cDNA molecule is a laboratory-made version of a gene that contains only its information-rich regions; these molecules provide a way for genome researchers to fast-forward through the genome to biologically important areas.

cDNA molecules are made using molecules of RNA (similar to DNA) obtained from living cells. In the cell, expression of the information from DNA into a protein first requires transcription of DNA into nuclear RNA molecules. These nuclear RNAs have noncoding regions that are processed out in the course of forming cytoplasmic RNAs (messenger RNAs). Because mRNAs are too fragile to withstand laboratory manipulations, scientists make sturdy double-stranded copies called complementary (or copy) DNAs, or cDNAs.

All DNA clones derived from a particular tissue constitute a library of clones representing the genes that were expressed when the source tissue was harvested. The analysis of libraries from many different tissues, obtained under a variety of physiological conditions, will be necessary to decipher the organ-specific patterns of gene expression.