BioGPS was initially created as a mechanism to disseminate reference gene expression data sets online of human and mouse. In particular, many diverse mouse and human tissues mouse tissues were systematically profiled using Affymetrix tiling arrays. BioGPS has grown to now a gene based portal, which can be customized to different types of data. In SlideBase, we have focused on human expression atlas, specifically from the U133 Affymetrix chip. In SlideBase, the unit that is searchable are affymetrix probe IDs, usually corresponding to annotated genes, and associated with the gcrma-normalized expression data from that probe.