Production of altered cell foci in tissue culture by defective Moloney sarcoma virus particles.

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Abstract
Moloney sarcoma virus (MSV) i?duces focal areas of altered cells in mouse tissue cultures. MSV preparations consist of a mixture of focus-forming and non-focus-forming particles, with the latter in large excess. Focus production generally follows a two-hit dose response curve, but when MSV is titrated in the presence of excess Moloney leukemia virus, the number of foci is increased, and the dose-response relation assumes a one-hit character. It is concluded that focus formation is initiated by a cell dually infected with focus-forming and non-focus-forming particles, the focus-formers being defective. Their defectiveness differs from that of Rous virus in that they require helper for initiation of a focus, while the Rous virus requires helper for virus growth but not forfocus production.