Where does the term "cos site" come from?
&quot;Cos&quot; is the abbreviation of &quot;cohesive end site&quot;. This is a speciality of the lambda phage which has to be linearized to fit into the phages head, but circularizes in the host cell. To achieve this, it has the cohesive (or sticky) sites at each end.
See here and also the publication by Allan Campbell on &quot;Bacteriophages&quot;.