THOMAS et al.1 have constructed a derivative of phage λ (λgt·λC) that is useful for cloning DNA fragments 1,000 to 15,000 base pairs long from other organisms. Their cloning procedure involves cutting the phage DNA into three fragments with EcoRI nuclease, removing the central (or "C") fragment, and annealing the outside fragments with an RI digest of "n DNA. We have recently introduced three amber mutations - Wam403, Eam1100, and Sam100 - into λgt·λC with the object of making the phage less likely to encounter a susceptible host in nature 2. The NIH Advisory Committee on Recombinant DNA Research, on the basis of appropriate tests, has certified λgtWES·λC as an EK2 vector3,4.
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