Identification of Differentially Expressed Genes in Sorted Cell Populations by Two-Dimensional Gene-

Differential activity of genes is one of the major mechanisms underlying a vast array of biological phenomena. Classical genetic approaches (from phenotypes to genes) have proven their exquisite potential for dissection of complex signaling pathways regulating the development of organisms and the functioning of individual cells. In recent years, with the advent of a number of techniques for studying gene function, the reverse genetics approach (from genes to phenotypes) has received broad acceptance. One of the advantages of this strategy is that it makes genes, whose dysfunction either does not produce an evident phenotype or is lethal, amenable to analysis. Reverse genetics has spearheaded the development of procedures for identification of candidate genes for this type of analysis by detecting spatial or temporal changes in geneexpression patterns. A significant range of methods have been proposed (1 –7 ); in particular, the advent of microarray hybridization techniques promises to increase gene-expression analysis throughput by two or more orders of magnitude (8 ,9 ). Some of these procedures have been used to identify genes expressed differentially during hematopoiesis (10 ,11 ).