Characterization of an endogenous retrovirus-repetitive DNA chimera in the mouse genome

We found that an endogenous mouse mammary tumor provirus, GR-MTV-8, is embedded within a member of the BAM HI family of long interspersed repetitive mouse DNAs. GR-MTV-8 appears to be transcriptionally silent at its normal chromosomal position in the mouse genome. The provirus is transcriptionally active, however, when cloned and transfected into mouse cells (Kennedy et al., Nature (London) 295:622-624, 1982). We propose that the transcriptional inactivity of GR-MTV-8 in situ is due to an inhibitory effect, possibly involving DNA methylation, attributable to the flanking BAM HI element.