Developing genome maps was a key step to prepare for DNA sequencing, but the increasingly detailed maps have also been an important tool orienting hundreds of geneticists hunting for disease genes.

With enough markers in place, the Human Genome Project scientist created "libraries" of clones that span the genome. Each of the clones contains a manageably small fragment of human DNA that is stored in bacteria. Scientists can tell what part of the human genome each clone derives from by figuring out what markers each contains.

This clone-by-clone approach for analyzing the human genome makes it possible to double check the locations of sequence. And because the Human Genome Project has been an international effort with many laboratories taking part, carving up the genome has allowed different groups to coordinate their work effectively.