Matrix attachment region binding proteins have been shown to play an important role in gene regulation by altering chromatin in a stage-and tissue-specific manner. Our previous studies report that SMAR1, a matrix-associated protein, regresses B16-F1-induced tumors in mice. Here we show SMAR1 targets the cyclin D1 promoter, a gene product whose dysregulation is attributed to breast malignancies. Our studies reveal that SMAR1 represses cyclin D1 gene expression, which can be reversed by small interfering RNA specific to SMAR1. We demonstrate that SMAR1 interacts with histone deacetylation complex 1, SIN3, and pocket retinoblastomas to form a multiprotein repressor complex. This interaction is mediated by the SMAR1(160-350) domain. Our data suggest SMAR1 recruits a repressor complex to the cyclin D1 promoter that results in deacetylation of chromatin at that locus, which spreads to a distance of at least the 5 kb studied upstream of the cyclin D1 promoter. Interestingly, we find that the high induction of cyclin D1 in breast cancer cell lines can be correlated to the decreased levels of SMAR1 in these lines. Our results establish the molecular mechanism exhibited by SMAR1 to regulate cyclin D1 by modification of chromatin.The periodic movement of the cell cycle is orchestrated by programmed oscillations in the activity of cyclins, cyclin-dependent kinases (CDKs), and their target proteins, including pRb and E2F/DP1 complexes (34). In response to the mitogenic stimuli, normal cells exit G 1 phase and enter S phase by assembling a D type of cyclins with respective CDK partners. Cyclin D1, a G 1 -phase cyclin, belongs to a family of three closely related proteins termed cyclins D1, D2, and D3. These proteins are expressed in redundant fashion in all proliferating cells and collectively control cell cycle progression along with CDK4/CDK6. Cyclin D/CDK complexes further phosphorylate retinoblastoma (Rb) protein and release an E2F transcription factor that triggers progression into S phase. The activation of CDKs is dependent on their association with cyclin partners, while inactivation is dependent on CDK inhibitors. A fine control of cyclins and CDK inhibitors is set by both transcriptional and degradation mechanisms.A complex transcriptional regulatory mechanism has been shown to exist to coordinate the specific temporal profiles of cyclins (38). Previous studies have demonstrated that autoregulatory loops occur between CDKs and their substrate, cyclin D1 (17). Cyclin D1 is induced by several proteins in proliferative signaling and transformations, including Ras, Rac, and Stat5 (27, 41). Elevation of cyclin D1 mRNA in 50 to 70% of breast cancers, while failing to develop normal mammary glands in transgenic mice lacking both cyclin D1 alleles, associates its role in cancer as well as normal breast development (35,47). Aberrant cyclin D1 expression in the malignancies is attributed to gene amplification, loss of transcriptional control, and stabilization (23,40).The molecular mechanisms involved in cyclin D1 downregulat...