MAExplorer is a Java applet that runs in a user's Web browser. It allows the exploratory data analysis of quantitative cDNA expression profiles across multiple microarrays. Data may be viewed and directly manipulated in images, scatter plots, histograms, expression profile plots, cluster analysis and so on. A key feature is the clone “Filter” for constraining a working set of clones passing a variety of user specified tests. Reports may be generated with Web access to UniGene, GeneBank and other Internet databases for sets of clones found to be of interest. Reports may also be exported to MS-Excel. MAExplorer is being developed in a collaboration between the Laboratory of Genetics and Physiology (LGP, NIDDK) and the Laboratory of Experimental and Computational Biology (LECB,NCI). LGP has established a program designed to identify and understand genetic pathways operative during normal mammary gland development and tumorigenesis. One arm of this program focuses on the use of cDNA microarrays to profile gene expression patterns. For this purpose, cDNA (EST) libraries are generated, sequenced and clone inserts are spotted on nylon membranes. These membranes are used to monitor expression profiles under various physiological conditions. At this point, expression profiles have been obtained from several stages of normal mammary gland development and different tumour models. With this program, you may: (i) analyse the expression of individual genes; (ii) analyse the expression of gene families and clusters; (iii) compare expression patterns. Data is downloaded as required to the user's Web browser to perform real-time analyses on the user's computer. However, the user has access to the entire database and may save and share their explorations in a groupware environment. The MAExplorer may be accessed through the MGAP web site at