Hematopoietic neoplasias of the RFM/Un mouse contain somatic re-integration of the restriction endogenous ecotropic provirus

We have examined the spleen DNA of individual mice of the RFM/Un strain for evidence of re-integration of the endogenous ecotropic provirus in radiation-induced and spontaneous neoplasms. The ecotropic envspecific probe detects only a single 19 kb EcoRl or a single 7.0 kb HindIII fragment in all DNA preparations from normal tissues of RFM mice, corresponding to the endogenous provirus. Additional DNA restriction fragments containing the ecotropic virus (eco) specific sequence, corresponding to somatically acquired provirus, are detected in two out of five spleen DNA samples from animals with myeloid luekemia and one of three with thymk lym-phoma. In addition somatically acquired eco-specific fragments are also detected in >85% of DNA samples from reticulum cell sarcomas, a late occurring spontaneous hema-topoietk neoplasm in this mouse strain. These results are consistent with a ‘promoter/enhancer insertion’ model of leukemogenesis involving the endogenous ecotropic provirus and are of particular interest since the RFM/Un mouse possesses a locus that restricts exogenous infection of cells by the endogenous virus.