Chromids are elements that exist at the boundary between a chromosome and a plasmid, found in about 10% of bacterial species sequenced by 2009. These elements carry core genes and have codon usage similar to the chromosome, yet use a plasmid-type replication mechanism such as the low copy number RepABC. As a result, they have been variously classified as minichromosomes or megaplasmids in the past. In Vibrio, the bacterium synchronizes the replication of the chromosome and chromid by a conserved genome size ratio.