Massively parallel signature sequencing, MPSS, is an open platform that reveals the expression level of virtually all genes expressed in a sample by counting the number of individual mRNA molecules produced from each gene.The MPSS process involves cloning each mRNA molecule onto the surface of a 5 μm bead. The DNA combitag sequence is attached to a fragment of cDNA. The cDNA library is hybridized to beads. After hybridization, each of the beads displays amplified copies of one and only one starting mRNA molecule.MPSS has a routine sensitivity of a few molecules of mRNA per cell and the results are in a digital format that simplifies data management and analysis. MPSS results will be particularly useful for generating the type of complete data sets that will help to identify the functionally important genes in the sample of interest.