Bacterial artificial chromosome (BAC)
A length of DNA with sequences that allow it to be maintained and reproduced in bacteria. DNA libraries for genome sequencing can be made using BACs: the DNA of interest is inserted into the BAC, and the package is taken up by bacteria. As the bacteria grow and divide, the BAC and the DNA of interest is also copied. BACs were the cornerstone of sequencing the human genome; at the time, they could hold 100,000 - 200,000 base pairs of DNA (100-200 kb); now they can hold double or triple that amount.