Except for identical twins, each human has his or her own unique genome--the complete set of DNA, or genetic material, found in the 46 chromosomes of each cell. Scientists estimate that individuals differ in about 0.1% of their 3 billion DNA base pairs. Although people who make up a particular population group share common ancestors and are more likely to share some genetic sequences, scientists believe that individuals within a group are genetically more variable than the groups are.

Given these differences, all humans still share the same basic set of genes and genomic regulatory regions that control the development and maintenance of their biological structures and processes. The Human Genome Project's goal is to determine the DNA sequence for a complete 'reference' human genome that will help orient researchers and provide them with tools for further studies of fundamental human biology. Because the genome of each person is unique and different samples will be used for sequencing, the reference sequence will not represent an exact match for any one person's genome.

Some researchers outside the Human Genome Project are beginning to look more closely at differences in DNA sequences of particular genomic regions to study the role of genetic variation in disease and susceptibilities. Researchers in the Environmental Genome Project, for example, plan to sequence about 200 genes from 1000 individuals to investigate how some genetic differences influence susceptibility to environmental exposures. Another group seeks to catalog genetic differences in groups around the world (see the Human Genome Diversity Project). Through these and other future studies, scientists will begin to identify and understand factors influencing health.