The basic stages involved in (a) serial analysis of gene expression (SAGE) and (b) microarray technology. (a) SAGE is a sequence-based method of identifying differentially expressed cDNAs between two experimental samples. The technique involves the generation of gene-specific tags typically 10-14 basepairs in length . These tags are then ligated together to form di-tags that contain primer sites on each end to facilitate a polymerase chain reaction amplification step. The di-tags are subsequently ligated to each other to form concatamers of varying length, which are size fractionated and sequenced. The sequence of the individual 10-14 bp tag is then used to interrogate appropriate cDNA/EST (expressed sequence tag) databases to identify the specific gene in question unambiguously. (b) Microarrays, or chips, are arrays of oligonucleotides or cDNAs synthesized or spotted, respectively, onto glass or silicon slides in a predetermined spatial orientation. Total RNA is reverse transcribed, fluorescently labeled and hybridized to the microarray. The protocol for generation of probes and the type of labeling procedure varies depending on the type of array being used. Specific hybridization signals are detected by a fluorescent scanner, which facilitates the identification of the specific grid reference of the target sequence, and, therefore, target identification.