The Genome Project opened in 1990 with one of the most ambitious scientific goals ever conceived: the complete mapping of human DNA sequences. Accomplishing this goal heralded the Genetic Age. Each human has about 100,000 genes spread across 23 chromosomes. These genes consist of DNA sequences, a compound molecule composed of two of four molecules that are paired together in a linear sequence. Having mapped these sequences, scientists' understanding of human physiology grew, enabling them to separate genetic and environmental diseases and opening the door to powerful genetic therapies.