The twenty-first century heralds a new era for the biological sciences and medicine. The tools of our time are allowing us to analyze complex genomes more comprehensively than ever before. A principal technology contributing to this explosion of information is the DNA microarray, which enables us to study genome-wide expression patterns in complex biological systems. Although the potential of microarrays is yet to be fully realized, these tools have shown great promise in deciphering complex diseases such as cancer. The early results are painting a detailed portrait of cancer that illustrates the individuality of each tumor and allows familial relationships to be recognized through the identification of cell types sharing common expression patterns.