The diversity of vascular SM. Within the vasculature, the term smooth muscle cell is used to include any connective tissue cell that forms a coating around the endothelial tubes. These cells may have many different phenotypes, ranging from the typical muscular artery SMC, characterized by a dense filamentous network made of SMC-specific proteins, to cells with much less definitive phenotypes, such as the glomerular mesangial cell and the intralaminar cell of the internal mammary artery, which look more like fibrocytes and lack SMC-specific proteins. Recently, we have begun to realize that SM-like cells may even arise from endothelial cells or circulating precursors. The diversity of the promoter structure described in Figure 1, as well as the presence of non-CArG box promoters in other SMC-restricted genes, may reflect the diverse responses to injury required of the cells making up the vessel wall.