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Gene expression monitoring technology allows to study the expression patterns of thousand of genes in a variety of experimental conditions and contexts and produces large numbers of data requiring automatic methods for their analysis. The analysis of these data is at the heart of bioinformatics concerns (see http://ihome.cuhk.edu.hk/~b400559/arraysoft.html for a review of currently available software). The typical scenario for a bioinformatics research project has become a collaborative effort shared by several laboratories, producing, storing and analyzing common data in a coordinated way. To cope with this situation, bioinformatics web services seem a natural alternative to desktop-based applications since they provide universal access through a web interface from practically any web browser. (QJHQH (standing for Gene Engine) is a platform-independent web tool for exploratory analysis of gene expression data that aims at storing, visualizing and processing large sets of gene expression patterns. (QJHQH integrates a variety of analysis tools for visualizing, pre-processing and clustering expression profiles. The system includes different filters and normalization methods as well as an efficient treatment of missing data. A broad range of clustering algorithms and projection methods are also provided. Additionally, a Java suite for interactive analysis is also available. This tool enables the analysis of large sets of gene expression data in an easy and transparent way, allowing the study of the outcome of different pre-processing and clustering methods at the same time.