In molecular biology, a library is a collection of molecules in a stable form that represents some aspect of an organism. Two common types of libraries are complementary DNA (cDNA) libraries and genomic libraries. The nucleotide sequences of interest are preserved as inserts to a plasmid. A cDNA library represents all of the mRNA present in a particular tissue, while a genomic library represents all of the DNA sequences found in the genome, broken into fragments of a manageable size.