DNA chips, which identify strands of DNA by washing them over an array of samples on a substrate and allowing them to bind onto a sample with complementary sequence, let researchers tune in to the symphony of gene expression. They can, for the first time, see which of the multitude of genes in a cell is active at any one time, an ability that is already speeding the understanding of the functions of genes. And, although only a handful of products exist at the moment, DNA chips also look set to revolutionize drug discovery and diagnostics.