Serial Analysis of Gene Expression (SAGE) is a sequence-based approach that enables the comprehensive and quantitative analysis of gene expression within any cell type or tissue (1 ). The method is based on three main principles. First is the generation of a single 10-bp ‘tag’ sequence that contains sufficient information to uniquely identify a transcript, provided that the tag is obtained from a defined position within each transcript. Second is the concatenation of the tags, allowing the serial analysis of multiple transcripts in one single sequence run. Third is the quantification of the number of times a particular tag is observed in a certain SAGE tag population (SAGE library), which provides direct information on the expression level of the corresponding transcript.