A temperate bacteriophage that infects cells of the bacterium Escherichia coli, where it can either exist as a quiescent prophage (in a state called lysogeny) or undergo replication leading to lysis of the host cell and release of new phage particles. The phage particle consists of an icosahedral head, 64 nm in diameter, and a tail, 150 nm in length. The head contains the double-stranded DNA of the phage genome. Lambda phage has been intensively studied as a model of viral infection and replication and is much used in genetic research and in genetic engineering. Modified lambda phages are used as vectors in gene cloning, especially for packaging relatively large amounts of foreign DNA.