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Crude extracts of bacteria lysogenic for temperate phage Mu contain proteins that retain specifically Mu DNA on nitrocellulose filters. The amount of binding protein is directly proportional to the number of Mu prophages per E. coli genome. Specificity of the binding reaction could be demonstrated by using heterologous DNAs as substrate and by a competition experiment. By using hybrid plasmids containing different amounts of the immunity end and extending to various degrees into Mu DNA, it was found that the binding activity is coded for by the left 1,000 nucleotide-pair HindIII fragment. When using these hybrid plasmids as binding substrate, two different binding sites for the immunity product were detected. Joining of the MucI gene to the left ? early promoter resulted in increased production of immunity protein at elevated temperature. A possible explanation for the relatively low amounts of immunity protein in all of the different strains studied is discussed.