The dedifferentiation and the abnormal growth of vascular smooth muscle cells (VSMC) is a major component of vascular diseases. Hexamethylene-bis-acetamide (I—lMBA) has recently been shown to modulate the differentiation and growth of VSMC in viz‘r0 and in viva. To determine the mechanism of this effect, we treated human coronary artery VSMC with HMBA and analyzed the effect of HMBA on gene expression by mRNA differential display. HMBA modulated the expression of a number of specific message fragments. By Marathon extension and cDNA screening, we isolated a novel gene, HEXIM1, that is up-regulated in HMBAstimulated VSMC. HEXIMI cDNA has a length of 3624 bp and encodes for a protein of 359 amino acids. Northern blot analysis demonstrated that the level of HEXIMI mRNA rapidly increased at 1 h after HMBA stimulation and was sustained for 48 h. These results suggest that l—lEXlM1 plays an important role in the regulation of the growth and differentiation of VSMC. Vascular smooth muscle cells (VSMC) are usually confined to the media of the vessel wall and maintain vascular tone by their contractile functions. However, abnormal proliferation of VSMC in the vessel intima is a major component of vascular diseases including atherosclerosis, vascular rejection, and restenosis following angioplasty (23, 26). It has been suggested that the contractile cells in the media are differentiated, whereas the proliferative cells in the intima become dedifferentiated (1, 2, 20). This proliferative phenotype of Correspondence to: Dr Masatoshi Kusuhara at the above address. Telephone: 81-45-753-2500 Fax: 81-45-753-2879 E-mail: mkusu@aurora.dti.ne.jp Sequence data from this article have been deposited with the DDBJ/EMBL/GenBank Data Libraries under Accession No. AB02ll79 VSMC is characterized by a reduction in the volume fraction of myofilaments, an increase in the proportion of synthetic organelles, the loss of contractile protein, and secretions of many proteins such as growth factors and extracellular matrix (17, 25). However, the precise molecular mechanisms underlying this process remain unclear. A differentiating agent, hexamethylene-bis acetamide (HMBA), has been shown to cause morphological and functional differentiation in several transformed cell lines and to inhibit cell proliferation in vz'z‘r0 and in vivo (22, 29). Recently, it was reported that HMBA inhibits the proliferation and induces the differentiation of VSMC in viz‘r0 (5, 8). Furthermore, we previously reported that HMBA prevented neointimal formation in a rat carotid balloon injury model (7). Evidence is accumulating that HMBA modulates the