A 12-nucleotide bases-long segment of single stranded DNA that exists at both ends of the bacteriophage lambda's double-stranded genome. The two cos sites at the ends of the genome are complementary to one another so that the genome can become circular once the virus has infected a host bacterium. The circular genome can then be duplicated continuously until there are many repeats of it strung together; the cos sites show the virus where to cut them apart right before they are packaged into individual capsids as new progeny viruses ready to infect more host cells.