cDNA clones containing DNA sequences of mRNA transcripts are essential resources for annotating the protein-coding genes of a genome as well as the expression of their protein products. A cDNA library is a collection of multiple cDNA clones, which in turn can be organized by species, tissue source or by gene family. Vectors containing cDNA also vary- some are expression-ready, in purified plasmid format with SV40 and CMV promoters for expression in mammalian systems. Some suppliers provide EST, or expressed sequence tag libraries which arise from smaller sections of cDNA, representing smaller portions of expressed genes. Some things to keep in mind include the type of vector harboring the cDNA library, and the sample format, which may be purified DNA, or bacterial glycerol stock.