Endogenous C-type virus of a mouse cell line and its defectiveness.

MLg cells originating from a newborn ddY mouse began to release C-type virus by treatment with 5-iodo-2'-deoxyuridine (5 mug/ml) for 48 h, or spontaneoulsy after prolonged cultivation. The data indicated the presence of three categories of the viruses spontaneously induced, i.e., virus equipped with the abilities of infecting and transforming the S+L- C182 cells, virus which can infect C182 cells but is unable to transform C182 cells, and virus which cannot infect nor transform the C182 cells, but is positive for the XC test. The type of the virus produced depended upon the cell clone. With the advance in the passage level, the cell population was dominated by the cells producing, uniquely, the last category of the virus.