MPSS (Massively Parallel Signature Sequencing) is a sequencing-based technology that uses a unique method to quantify gene expression level, generating millions of short sequence tags per library. The Plant MPSS databases are the largest publicly available set of tag-based gene expression data. These databases provide MPSS data with a user-friendly interface for four organisms (Arabidopsis, rice, grape, and Magnaporthe grisea, the rice blast fungus). These data include mRNA data and we’re currently expanding a set of small RNA data. The databases are built on a common set of web interfaces and are equipped with various graphical and analytical tools that allow the user to retrieve and analyze the data. The sequencing depth provides a quantitative assessment of transcript abundance (i.e. the expression level of the corresponding gene), while greatly increasing the likelihood of discovering novel transcripts. The website, equipped with various graphical and analytical tools, is available at http://mpss.udel.edu