Fosmid clones are similar in size (ca. 40kb) to cosmids, but, like BAC clones, they contain replicons derived from the F factor for DNA replication and segregation. Because of this property, they are present in low copy number and consequently more stable than cosmids. Also, the copy control vector can be used for easy DNA purification from fosmid clones.
Fosmid libraries are commonly used for closing gaps in a whole genome sequence, physical maping, metagenomic and expression screening and more. Although we greatly prefer to perform the DNA isolation at our facilities we understand that this is not always feasible. Fosmid libraries, contrary to BAC large-insert libraries, do not require HMW DNA (typically 2Mb) as a starting material. This allows scientiststhe needed flexibility in providing DNA to construct a library.