 As more and more diseases were being linked up to locations in the human genome throughout the 1990s, geneticists shifted their attention to the grand prize—sequencing the entire human genome (Cook-Deegan 1994). Ever since the discovery of the double-helical structure of DNA, geneticists knew what genes were made of and what structure they took. But they did not know how many genes there were. And with the exception of the handful of genes that had already been discovered, they did not know where the vast majority of genes were located. The challenge was to spell out the ordered sequence of the human genome in its entirety—no small task when you consider that all three billion base pairs of the human genome reside in cells no bigger than the period at the end of this sentence. The Human Genome Project met the challenge with the largest and most expensive biological collaboration in history. An international consortium of scientists at twenty different sequencing centers spread out across the United States, the United Kingdom, France, Germany, Japan, and China took charge of different portions, breaking up the entire genome into smaller and smaller overlapping segments until stretches several thousand base pairs at a time could be sequenced. Then the whole thing was pasted back together using the areas of overlap as guides. The Human Genome Project was officially completed in 2003 (entry:  the Human Genome Project).  