Abstract
Conclusion: Mouse mammary tumour virus has been extremely useful for the investigation of steroid hormone activation and the early steps of mammary tumorigenesis. We have dealt with some of the molecular mechanisms: transcriptional regulation and its correlation with DNA modification, glucocorticoid regulation of transcription and the hormone-independent enhancement of genes in the vicinity of proviral DNA. The expression of MMTV in many cell types from different species, infected or transfected in vitro, contrasts with the strong tissue specificity of MMTV expression in vivo. The investigation of the determinants of tissue specificity of MMTV expression will be of importance in the future.