The endogenous mouse mammary tumour virus locus Mtv-8 contains a defective envelope gene.

The Mtv-8 associated provirus, GR40, is not expressed in vivo. However, upon transfection into rat XC cells, the two MMTV specific mRNAs of 35S and 24S are transcribed. Further, the level of these transcripts is augmented when the transfected cells are grown in the presence of dexamethasone (Ponta et al., 1983). No virus can be detected in the medium of the transfected cells. Intracellular protein analysis of these transfected cells shows that although apparently authentic gag proteins are synthesized, a novel protein of 68 kDa is the only env related protein detectable. The sequence of the env gene of GR40 was determined and the predicted amino acid sequence of the env protein obtained. A premature termination codon is present 68 amino acids before the COOH-terminus of the known MMTV env precursor Pr73env. This would result in an env protein of about 68 kDa, in agreement with the size of the env protein found in GR40 transfected cells. The lack of the carboxy terminus may be responsible for the non-processing of the aberrant env precursor protein, p68.