Genomic and cDNA libraries are collections of DNA fragments used for gene discovery and analysis. cDNA libraries contain only expressed genes and are useful for eukaryotic analysis since they lack introns. Genomic libraries contain all DNA sequences from an organism's genome. Both types of libraries are constructed by fragmenting DNA, inserting fragments into cloning vectors, and transforming bacteria to generate clones containing DNA fragments. Libraries are screened using probes to identify clones containing genes of interest.