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The Mouse Mammary Tumor Virus (MMTV) is expressed at the highest levels in the mammary glands of lactating mice. It has been shown that several milk proteins including beta-casein are dependent upon the presence of extra-cellular matrix (ECM) for optimal expression. The analysis of the beta-casein gene was done using a cell line (CID9) that can differentiate when cultured in the presence of ECM and lactogenic hormones. In this study, we analyze gene transcription of full length MMTV LTR in CID9 cells and show that it is upregulated in the presence of ECM and the effect is independent of activation by hydrocortisone. The ECM-responsiveness occurs only when the DNA is stably transfected which suggests that chromatin structure may be important We show using a deletion series that the maximal ECM response is seen with the MMTV minimal promoter. Since the ECM-responsiveness occurs only when the LTR is stably integrated we analyzed the effect of deacetylase inhibitors. LTR gene transcription is inhibited 60% by sodium butyrate and 80% by Trichostatin A and this inhibition is independent of hydrocortisone and ECM Additional evidence shows that the 5' 25 base pairs of the LTR is required for high expression in CID9 cells and its regulation by chemical inhibitors of histone acetylation.