There is a new tool for the proteomics era—a cell-based microarray that can be used as a living screen of gene function. Researchers at the Whitehead Institute for Biomedical Research (Cambridge, MA) grew human embryonic kidney (HEK 293) cells on a slide printed with an array of complementary DNAs (Nature 411, 107–110, 2001). Cells growing on each “spot” take up the cDNA vector and express the encoded protein. The researchers could then use standard detection techniques such as autoradiography or in situ hybridization to study the expression, and even the subcellular location, of the protein. The cell microarrays have many advantages over existing protein chips: there is no need for time-consuming protein purification, and the cDNAs are more stable than proteins. The cells also provide an ideal environment in which to study membrane-bound proteins, and can be used to monitor the transient expression of proteins. Eventually, when cDNAs are available for the estimated 30,000 genes in the human genome, a pan-genomic cell array could fit on just a handful of cell chips. David Sabatini, the lead author, says that the team's next goal is to expand the number of cell lines that will work in the system, and to make arrays involving a loss of gene function—for example, using antisense technologies.