FIGURE 1 (a) The structure of the altered bacteriophage λ substrate developed by Nash (λ attB-attP), and the reaction promoted by the recombination system. A recombination reaction between the attP and attB sites produces two slightly different sites called attL and attR (products λ attL and λ attR). (b) The number of recombinant λ phages produced (as percentage of total) as a function of incubation time with the enzyme extract.