With the exception of identical twins, no two human genomes are exactly the same. But how different are they? Are two humans more alike than two orangutans? Dr. Lander explores human genetic variation and how it may affect individual susceptibility to certain diseases. A collaborative research effort is now cataloging all common human genetic variations to help us better understand disease mechanisms. Dr. Lander also discusses a powerful tool—the DNA microarray—that can be used to measure the expression level of many genes simultaneously. DNA microarrays, for example, have been helpful in diagnosing different tumors. The widespread use of microarray technology promises to guide research into specific genes and proteins associated with many diseases.