Lambda dv (hdv) is a 515% fragment of the bacteriophage A genome including those genes and genetic sites that act in the process of replication and its regulation. The hdv genome is in the covalently closed circular form and is perpetuated in the plasmid state in its host, Escherichia coli K12. The copy number is about 20 to 100 per chromosome and the multicopy DNA molecules replicate in accordance with the random-choice mechanism of Rownd (1969; Matsubara and Mukai, 1975). Since this plasmid is derived from A, whose genetic structure has been intensively studied, and since, as will be shown below, one can derive new mutant Adv plasmids from A carrying known mutations, the Adv plasmid is an ideal system for the study of genetic mechanism of replication control. The first isolate of the Adv series, later named Advl, was discovered in 1968 by Matsubara and Kaiser (1968) in Stanford among A-resistant survivors in a culture of E. coli infected with Avir phage. Most of such survivors were bacterial mutants that do not allow growth of A (i.e., gro mutants: for example, see Georgopoulos and Herskowitz, 1971). In addition to these, there were some bacterial clones whose tolerance to A was unstable and which “reverted” to a A-sensitive state at high frequency. One such strain was studied in detail and was found to carry an unstable plasmid which was responsible for the A tolerance, Subsequently, the plasmid was found to be much more stable in recA host bacteria (Matsubara, 1972a; Hobom and Hogness, 1974), for reasons that remain unclear. However, this has enabled us to study several characteristics of this plasmid. Advl arose spontaneously by an illegitimate recombination event(s), deleting most of the A genome. Marker rescue studies have shown that only a continuous region extending from pLoL through the gene P (Fig. 1) has been retained. The fact that this fragment can replicate implies that all the genes and sites necessary for replication (the drive unit for replication) are located in this region. This is consistent with the general situation in bacteriophages and plasmids, where genes for related functions tend to cluster. Advl is thus an extract of this drive unit from the A genome. Conversely, replication of the bacteriophage A genome may be regarded as being carried out by the drive unit of the Adv plasmid. It has been possible to prepare mutant Adv’s by repeating essentially the same procedure whereby Advl was isolated. Thus, using an appropriate A mutant one can obtain Adv carrying that same mutation. It was necessary to start with phage that is not able to lysogenize (e.g., those mutated in the repressor gene c1 or in the operator (Matsubara, 1974a; Matsubara, 1976)). Ultraviolet irradiation of the phage prior to infection increases the frequency of Adv (Berg, 1974a). One obtains about 500 Adv-carrier clones by mixing 2 x IO* particles of uv-irradiated A with 4 x lo* cells. This figure is lowered by a factor of IO100 upon omission of the uv irradiation. The illegitimate recombination that gives rise to Adv occurs in the absence of the ret A function, but the details are not known. Once the autonomously replicating fragment appears, it presumably multiplies until the progeny particles “saturate” the cytoplasm and the other, nonreplicating, segment of A disappears. The newly arisen