To provide standardised description of gene expression and cross platform querying of databases, we have developed eVOC (http://www.sanbi.ac.za/evoc/), consisting of four orthogonal ontologies which describe Anatomical System, Cell Type, Pathology and Developmental Stage. We have annotated 47 microarray expression data sets and all publicly available human cDNA and SAGE tag libraries. eVOC has been integrated with the public resource EnsMart, which provides linking of transcripts and libraries with expression terms and the human genome sequence (http://www.ensembl.org/Homo_sapiens/martview).