Abstract: Each cell normally has a widely differing number of mRNA transcribed for each gene. Consequently, a full-length cDNA library constructed from the mRNA would also have a widely differing number of cDNA for each gene. A normalized library of the full-length cDNA of a cell is useful for basic, applied, industrial, and medical research. This invention provides for a method for constructing a normalized full-length cDNA library by probing the members of a non-normalized cDNA library with a library of probes generated from mRNA in order to identify the cDNA of genes that have low or high expression. A collection of the cDNA from the library of the genes that have low expression would constitute a normalized library of these genes. This invention also provides for a method to reduce the number cDNA of genes that have high expression represented by probing these cDNA with a library of probes generated from a small randomly selected number of these cDNA.