The completion of the human genome and the genomes of several model organisms is fundamentally changing how biological discovery takes place. More than any specific benefit, it has already changed how biological questions can be addressed. This is best illustrated in the application of deoxyribonucleic acid (DNA) microarray technology, by which thousands of gene segments are deposited on a glass slide or chip and then used to simultaneously analyze the expression levels of the corresponding genes. Thus, instead of testing the changes in expression of only a few genes at a time, it is now possible to measure changes in tens of thousands of genes in a single experiment. Having the complete blueprint of an organism in its DNA sequence is opening up possibilities at almost every level, including protein function.