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The phage λ (lambda) is a temperate phage that lives in E. coli. Once the phage is inside its host, it may integrate itself into the host's DNA. In this state, λ is called a prophage and stays resident within the host's genome, without causing it much harm. This way, the prophage gets duplicated with every cell division of the host. The DNA of the prophage that is expressed in that state codes for proteins that look out for signs of stress in the host cell. Stress can be a result of starvation, poisons (like antibiotics), and other factors that can damage or destroy the host. At that point, the prophage becomes active again, excises itself from the DNA of the host cell and enters its lytic cycle. The reactivated phage takes apart the host's DNA and "reprograms" its "protein factory" to produce new phages in multiple copies. When all resources of the host are depleted from building new phages, the cell is lysed (the cell membrane is broken down), and the new phages are released.

The integration of phage λ takes place at a special attachment site in the bacterial genome, called att. The sequence of the att site is called att and consists of the parts B-O-B', whereas the complementary sequence in the circular phage genome is called att and consista of the parts P-O-P'. The integration itself is a sequential exchange (see genetic recombination) via a Holliday structure and requires both the phage protein int and the bacterial protein IHF (integration host factor). Both int and IHF bind to att and built an intrasome, a DNA-protein-complex designed for site-specific recombination of the phage and host DNA. The original BOB' secuanes is changed by the integration to B-O-P'-phage DNA-P-O-B'. The phage DNA is now part of the host's genome.