SUMMARY

The structure and sites of integration of proviral DNA were studied in 19 clonally related Kirsten murine sarcoma virus-transformed non-producer NIH/3T3 cell lines. The majority of these cell lines contained a single provirus, inserted colinearly with respect o unintegrated linear viral DNA, and lacking detectable methylation at MspI/HpaII sites. Although all proviruses were located at distinct integration sites in the host cell genome, the possible xistence of similarities between some adjacent host flanking sequences, uggested from restriction mapping data, could not be ruled out. In three phenotypically reverted cell lines no change in either proviral DNA or adjacent host flanking sequences wa detectable. In addition, the revertant proviruses lacked detectable methylation at Mspl/HpaII sites. These findings suggest hat changes in cellular function(s) may be responsible for loss of transformed phenotype in these cells