Recent suppressive subtractive hybridization analysis on human atherosclerotic plaque-derived RNA revealed genes upregulated in plaques with a thrombus versus stable plaques. Clone SSH6, containing part of a putative open reading frame of an unknown protein, was further investigated. Full-length cDNA, coding for a 473-amino acid (aa) protein, was identified in a vascular smooth muscle cell (SMC) cDNA library. Bioinformatics suggested the presence of multiple SSH6 variants due to alternative splicing of exon 3. Multiple-tissue Northern blot analysis demonstrated a differential expression pattern of these variants, as a ubiquitously expressed SSH6 mRNA missing exon 3, was detected apart from a putative vascular SMC-specific form containing exon 3. Western blot analysis indicated a ubiquitous 35-kDa protein (SSH6-beta), in addition to a 45-kDa protein (vasculin), detected in the vascular wall and in plasma. Analysis of arteries displaying various stages of atherosclerosis indicated that the vasculin/SSH6-beta ratio increases throughout atherogenesis. Immunohistochemical analysis demonstrated cytoplasmic expression of SSH6 gene products in macrophages, endothelial cells, and SMCs. In summary, we identified a novel mRNA/protein, vasculin, in the arterial wall and plasma. The regulated expression of vasculin in plaques suggests a role in atherogenesis. Moreover, its presence in plasma opens perspectives for vasculin as a marker for atherosclerosis.