Unlike plasmids, they can also be packaged in vitro into phage capsids, a step which requires "cohesive ends", also known as "cos" sites also used in cloning with a lambda phage as a vector, however nearly all the lambda genes have been deleted with the exception of the "cos" sequence. The hybrid cosmid DNA in the capsids can then be transferred into bacterial cells by transduction. Since there is a requirement for in vitro packaging whereby at least 38 kb of DNA is required between the cos sites, the vector without insert DNA will not be packaged ( Plasmids instability is increased if the novel inserted DNA contains many direct repeats or palindromic (inverted repeats) DNA. This instability can largely be counteracted by using a host bacterium with specific mutations affecting DNA recombination (N.B. Absence of inverted repeats was noted in the first Hohn & Collins publication cited above; see also).