The characteristic of H5ts125, a temperaturesensitive DNA-minus mutant, to transform 3 to 8 times more rat embryo cells than wild-type 5 adenovirus was correlated with the persistence of an increased proportion of the viral genome in cells independently transformed at the nonpermissive (39.50) or semipermissive (360) temperature. Reassociation (3)(4)(5)(6). Hence, the isolation and characterization of temperature-sensitive mutants of adenoviruses (7-10) suggested that they, too, may be useful to identify the critical gene(s) involved in, and to detect the gene product(s) essential for, transformation. In contrast to our expectation from data obtained with Rous sarcoma (1) and SV40 (2) viruses, however, an adenovirus mutant defective in DNA replication (5), H5ts125, was discovered that transformed rat embryo cells at frequencies 3 to 8 times greater than wild-type 5 adenovirus at the nonpermissive (39.50) and semipermissive (36°) temperatures (11,12). This finding implies that a specific viral gene product affects transformation, perhaps by modulating against it, so that, when the viral protein is nonfunctional or functions aberrantly, transformation is less restricted. A single-strand specific DNA-binding protein has been identified as the viral protein encoded in the ts125-mutated gene (13)