Bacteriophage is a virus which infects a bacterium and which is many times used in molecular genetics experiments as the vector, or cloning vehicle. Recombinant phages can be made in which certain non-essential l DNA is removed and then replaced with the DNA of interest. The phage can accommodate the DNA "insert" of about 15-20 kb. Replication of that virus will hence replicate the investigator's DNA. One would take use of phage l rather than a plasmid if the desired piece of DNA is rather large. Bacteriophage l and M13 are the ones usually used in cloning or subcloning of small genes or DNA fragments in E. coli. Bacteriophage P1 is one which is used for fragments up to 95 Kb in size.