Publisher Summary This chapter describes the characterization of the early molecular events that lead to the induction of prophages and SOS functions. The chapter also obtains preliminary evidence that specific oligodeoxynucleotide, such as d(A-G-G), inactivate phage (φ80 and λ) repressors in the permeabilized cells. This strongly suggests that specific oligodeoxynucleotide or single-stranded DNAs with specific base sequences are involved in the induction of SOS functions. Permeabilized and biologically active E.coli cells were obtained by plasmolysis and incubation of the cells in a reaction mixture that allows protein synthesis. These cells become permeable to low-molecular-weight proteins and oligodeoxynucleotide. Using this system, one studied two types of prophage induction. One is the induction by low-molecular-weight DNAse, such as micrococcal and pancreatic DNase, and the other by specific guanine-containing oligodeoxynucleotide. Prophage induction by the DNAse was dependent on a functional recBC-DNase and degradation of the DNA by the enzymes. On the other hand, the oligodeoxynucleotide induction was independent of recBC-DNase and did not require degradation of chromosomal DNA. There are two classes of guanine-containing deoxy oligonucleotides that are effective in prophage induction. One is specific deoxydinucleotides having the sequence A(or G or 1)-G; the other is oligodeoxyguanylates with a 5’ phosphate and a chain length between 6 to 18.