Complementary DNA (cDNA) libraries offer certain advantages over genomic libraries and continue to be particularly useful methodology for gene cloning. Complementary DNA library contained cDNA copies called cDNA, which are made from mRNA molecules of a cell population and therefore represent the genes being expressed in the cells at the time that library was made. cDNA is complementary to the nucleotides sequences of the mRNA. cDNA libraries have been particularly useful for identifying and studying gene expressed in certain cells or tissues in certain conditions. For example, during development, cell death, cancer, and other biological processes cDNA was used. Reverse transcriptase uses the mRNA as template to synthesize a complementary DNA strand and forms a cDNA. cDNA is useful for cloning eukaryote gene in prokaryotes.