An approach to determination of the sequence of a large, randomly selected, piece of genomic DNA. A cosmid that contains the DNA is sonicated into smaller fragments of 1.0-1.5kb, which are ligated into a vector (e.g. M13) containing a universal primer, and each fragment is sequenced. In order to obtain the complete linear structure of a 40kb cosmid, 800-1000 fragments must be sequenced. More recently, bacterial artificial chromosomes (BACs) have been employed, allowing up to 350kb fragments to be cloned for sequence analysis.