The Human Genome Project documented our genetic sequence and discovered everyone's DNA to be 99.9% identical. However, even very small differences in DNA sequence can have very big effect on health and disease.

The same gene may be working properly in one person, but a small mutation could cause it not to work at all in another.

To find those disease-causing mutations, researchers use microarrays to genotype a patients' DNA and determine the exact sequence — A, T, C, or G — for thousands and thousands of single nucleotide polymorphisms (SNPs) distributed throughout the genome.