A method for comparison of different organs, or physiological or disease states by the genes that are uniquely expressed in them. The mRNAs of two types of cell to be compared, e.g. normal and cancer cells, are isolated, differently labelled for detection and hybridised to an appropriate cDNA library, preferably a normalised library. The library can be in the form of clones blotted onto duplicate philtre membranes and the two sets of mRNAs, or can be labelled with two different radioisotopes, so that a clones that are equally labelled on both membranes identify a gene that is expressed equally in both kinds of cell, and a clone that fails to appear on one membrane will indicate transcription in only the other type of cell. A more sophisticated version labels the mRNAs with two different fluorophores, e.g. red emission for cancer cells and green emission for normal cells, so that when they are hybridised to the cDNA library that is arrayed in a relative excess on a single glass surface, the hybridization of both fluorophores to a single cDNA fragment will appear yellow, while the hybridization of only one, e.g. the normal cell mRNA, will appear green.