P1 is a temperate bacteriophage that infects Escherichia coli and some other bacteria. When undergoing a lysogenic cycle the phage genome exists as a plasmid in the bacterium unlike other phages (e.g. the lambda phage) that integrate into the host DNA.  P1 has an icosahedral head containing the DNA attached to a contractile tail with six tail fibers.
The P1 phage has gained research interest because it can be used to transfer DNA from one bacterial cell to another in a process known as transduction.  As it replicates during its lytic cycle it captures fragments of the host chromosome.  If the resulting viral particles are used to infect a different host the captured DNA fragments can be integrated into the new hosts genome.  This method of in vivo genetic engineering was widely used for many years and is still used today, though to a lesser extent. P1 can also be used to create the P1-derived artificial chromosome cloning vector which can carry relatively large fragments of DNA.  P1 encodes a site-specific recombinase, Cre, that is widely used to carry out cell-specific or time-specific DNA recombination by flanking the target DNA with loxP' sites (see Cre-Lox recombination).