DNA microarrays and, more recently, protein microarrays, have become important tools for high-throughput genomic and proteomic studies. Transfected cell microarrays are a complementary technique in which array features comprise clusters of cells overexpressing defined cDNAs. Complementary DNAs cloned in expression vectors are printed on microscope slides, which become living arrays after the addition of a lipid transfection reagent and adherent mammalian cells. This article discusses two potential uses of cell microarrays in drug discovery: as a method of screening for gene products involved in biological processes of pharmaceutical interest and as in situ protein microarrays for the development and assessment of leads.