A bacteriophage (a virus which infects bacteria) that infects E. coli. It has a complex set of regulatory mechanisms to determine whether it will quietly insert its DNA into the bacterial genome to become dormant and to be reproduced whenever the bacterium reproduces (to lysogenize), or whether it will hijack the bacterium's cellular machinery to reproduce itself and prepare to infect more bacteria, causing the bacterium to self-destruct shortly after infection (to lyse). Lambda is particularly useful to geneticists because parts of it can be used to introduce foreign DNA into the bacterial genome; it is a cloning vector.