Gaps often remain after initial BAC contig construction.  These gaps occur if the Bacterial Artificial Chromosome (BAC) library screened has low complexity, meaning it does not contain a high number of STS or restriction sites, or if certain regions were less stable in cloning hosts and thus underrepresented in the library.  If gaps between contigs remain after STS landmark mapping and restriction fingerprinting have been performed, the sequencing of contig ends can be used to close these gaps.  This end-sequencing strategy essentially creates a novel STS with which to screen the other contigs.  Alternatively, the end sequence of a contig can be used as a primer to primer walk across the gap.