A hallmark of smooth muscle cell (SMC) phenotypic switching in atherosclerotic lesions is suppression of SMC differentiation marker gene expression. Yet little is known regarding the molecular mechanisms that control this process. Here we show that transcription of the SMC differentiation marker gene SM22α is reduced in atherosclerotic lesions and identify a cis regulatory element in the SM22α promoter required for this process. Transgenic mice carrying the SM22α promoter–β-galactosidase (β-gal) reporter transgene were crossed to apolipoprotein E (ApoE)^−/− mice. Cells of the fibrous cap, intima, and underlying media showed complete loss of β-gal activity in advanced atherosclerotic lesions. Of major significance, mutation of a G/C-rich cis element in the SM22α promoter prevented the decrease in SM22α promoter–β-gal reporter transgene expression, including in cells that compose the fibrous cap of the lesion and in medial cells in proximity to the lesion. To begin to assess mechanisms whereby the G/C repressor element mediates suppression of SM22α in atherosclerosis, we tested the hypothesis that effects may be mediated by platelet-derived growth factor (PDGF)-BB–induced increases in the G/C binding transcription factor Sp1. Consistent with this hypothesis, results of studies in cultured SMCs showed that: (1) PDGF-BB increased expression of Sp1; (2) PDGF-BB and Sp1 profoundly suppressed SM22α promoter activity as well as smooth muscle myosin heavy chain promoter activity through mechanisms that were at least partially dependent on the G/C cis element; and (3) a short interfering RNA to Sp1 increased basal expression and attenuated PDGF-BB induced suppression of SM22α. Together, these results support a model whereby a G/C repressor element within the SM22α promoter mediates transcriptional repression of this gene within phenotypically modulated SMCs in experimental atherosclerosis and provide indirect evidence implicating PDGF-BB and Sp1 as possible mediators of these effects.