Cosmids are predominantly plasmids with a bacterial "oriV", an antibiotic selection marker and a cloning site, but they carry one, or more recently two, cos sites derived from bacteriophage lambda. Depending on the particular aim of the experiment, broad host range cosmids, shuttle cosmids or 'mammalian' cosmids (linked to SV40 oriV and mammalian selection markers) are available. The loading capacity of cosmids varies depending on the size of the vector itself but usually lies around 40–45 kb. The cloning procedure involves the generation of two vector arms which are then joined to the foreign DNA. Selection against wild type cosmid DNA is simply done via size exclusion. Cosmids, therefore, always form colonies and not plaques. Also the clone density is much lower with around 105 – 106 CFU per µg of ligated DNA.