A different type of DNA library contains only the expressed sequences from a particular cell type. Such a cDNA library is constructed by isolating all the cell’s mRNAs and then copying them to DNA using a specialized type of DNA polymerase known as reverse transcriptase because it synthesizes DNA using RNA templates (Box 25-2). The complementary DNA (cDNA) molecules are then inserted into cloning vectors to form a cDNA library. A cDNA library can also be used to construct a DNA microarray (DNA chip), in which each different cDNA is immobilized at a specific position on a slide. A DNA chip can be used for detecting the presence of mRNA in a biological sample (the mRNA, if pres- ent, will bind to its complementary cDNA