Disclosed and claimed is a method for preparing a normalized sub-divided
library of amplified cDNA fragments from the
coding region of mRNAs contained in a sample. The method includes the steps of: a) subjecting the mRNA
population to reverse transcription using at least one cDNA primer, thereby obtaining first strand cDNA fragments, b) synthesizing second strand cDNA complementary to the first strand cDNA fragments by use of the first strand
DNA fragments as templates, thereby obtaining
double stranded cDNA fragments, c) digesting the
double stranded cDNA fragments with at least one restriction
endonuclease, the
endonuclease leaving protruding sticky ends of similar size at the termini of the
DNA after
digestion, thereby obtaining cleaved cDNA fragments, d) adding at least two adapter fragments containing known sequences to the cleaved cDNA fragments obtained in step c), the at least two adapter fragments being able to bind specifically to the sticky ends of the
double stranded cDNA produced in step c), the one adapter fragment being able to anneal to the primer having formula I in step f), the second adapter fragment being a termination fragment introducing a block against
DNA polymerization in the 5'->3' direction setting out from said termination fragment and the termination fragment being unable to anneal to any primer of the at least two primer sets in step f) during the molecular amplification procedure, the at least two adapter fragments being ligated to the cleaved cDNA fragments obtained in step c) so as to obtain ligated cDNA fragments, e) sub-dividing the ligated cDNA fragments obtained in step d) into 4n1 pools where 1<=n1<=4, and f) subjecting each
pool of ligated cDNA fragments obtained in step e) to a molecular amplification procedure so as to obtain amplified cDNA fragments, wherein is used, for an adapter fragment used in step d), a set of amplification primers having the general formula Iwherein Com is a sequence complementary to at least the 5'-end of an adapter fragment which is ligated to the 3'-end of a cleaved cDNA fragment, N is A, G, T, or C, the one primer having the general formula I where n1=0, and the second primer having the general formula I where 1<=n1<=4, the second primer being capable of priming amplification of any
nucleotide sequence ligated in its 3'-end to the adapter fragment complementary in its 5'-end to Com.