Analysis of rare messages in cDNA libraries is extremely difficult due to the substantial variations in the abundance of different transcripts in cells and tissues. Therefore, for rare transcript searches and analyses, the generation of equalized (normalized) cDNA is essential. Several cDNA normalization methods have been developed since 1990. A number of these methods have been optimized for the normalization of full-length enriched cDNA, and used in various applications, including transcriptome analysis and functional screening of cDNA libraries. One such procedure (named DSN-normalization) is based on the unique properties of duplex-specific nuclease (DSN) from kamchatka crab and allows the generation of normalized cDNA libraries with a high gene discovery rate.