The construction of full-length cDNA libraries allows researchers to study gene expression and protein interactions and undertake gene discovery. Recent improvements allow the construction of high-quality cDNA libraries, with small amounts of mRNA. In parallel, these improvements allow for the incorporation of adapters into the cDNA, both at the 5' and 3' end of the cDNA. The 3' adapter is attached to the oligo-dT primer that is used by the reverse transcriptase, whereas the 5' adapter is incorporated by the template switching properties of the MMLV reverse transcriptase. This allows directional cloning and eliminates inefficient steps like adapter ligation, phosphorylation, and methylation. Another important step in the construction of high-quality cDNA libraries is the normalization. The difference in the levels of expression between genes might be several orders of magnitude. Therefore, it is essential that the cDNA library is normalized. With a recently discovered enzyme, duplex-specific nuclease, it is possible to normalize the cDNA library, based on the fact that more abundant molecules are more likely to reanneal after denaturation compared to rare molecules.