Genomic DNA libraries represent the total complement of the genetic information of an organism’s DNA, as opposed to cDNA libraries, which contain only the protein encoding sequences expressed at a particular stage of the life cycle. Ideally, a genomic library contains the coding and control elements from all stages of the life cycle as well as a surfeit of sequence whose role in the storage and utilization of the organism’s heritable information is unknown. Comparison of the arrangement of sequences in the two types of libraries has been used as an approach in understanding the process of dispensing informational content to the cell. For example, the comparison has provided technique for identifying and understanding control elements for gene expression. Further, the genomic conformation of a segment of genetic material and its counterpart in the messenger RNA pool, from which cDNA libraries are constructed, may be different for a number of reasons. Features of the processing of RNA such as the excision of introns, RNA processing and exon splicing have been discovered because of the power of comparing genomic and cDNA libraries.