 To begin the project, researchers built maps of the human genome. They identified thousands of DNA sequence landmarks that helped them navigate across the chromosomes. Developing genome maps was necessary preparation for DNA sequencing. These same maps also served to orient geneticists who were hunting for disease genes. With enough landmarks in place, project scientists created libraries of clones that spanned the genome. Each clone contained a manageably small fragment of human DNA that was stored in bacteria. Scientists used the landmarks to tell them what part of the human genome each fragment came from. This clone-by-clone approach made it possible to double-check the location of each DNA sequence. It also allowed participating laboratories from around the world to carve up the genome and coordinate their work.