To sequence the human genome, Human Genome Project scientists cut relatively large clones called BACs, which are 100,000 to 200,000 bases long , into smaller fragments. The smaller fragments, which are about 2000 bases long, are typically stored in E. coli viruses.
The scientists determine the precise order of the larger clones, because pinpointing the positions of many smaller clones is much more work. But for actual sequencing reactions, the smaller clones are more suitable.
Human Genome Research Institute