Werner Arber, at the University of Basel, postulates in the late 1960s the existence of bacterial enzymes capable of cutting DNA at specific points, as a defense mechanism bacteria use to destroy the genetic material of invading viruses (bacteriophages) by recognizing sequences foreign to their own genome. Hamilton Smith, at Johns Hopkins University, isolates in 1970 the first of these enzymes — called EcoRI, from the bacterium Escherichia coli — and precisely determines the exact DNA sequence it recognizes and cuts, a specific six-base-pair sequence. Daniel Nathans, also at Johns Hopkins, applies these enzymes as a genetic-mapping tool, cutting the DNA of the SV40 virus into predictable, reproducible fragments to systematically study its genetic structure, demonstrating the enormous potential of these enzymes as tools for directed DNA manipulation in the laboratory. Restriction enzymes immediately become a fundamental and essential tool of genetic engineering: they allow DNA from any organism to be cut at precise, predictable points to insert genes of interest into other organisms, the base technique for producing recombinant human insulin, gene cloning, and virtually all molecular biotechnology developed from the 1970s onward.