Vascular smooth muscle cell (VSMC) hyperplasia plays an important role in both chronic and acute vascular pathologies. Considerable work has focused on the mechanisms regulating VSMC growth and the search for agents that could suppress VSMC hyperproliferation. One of the several inhibitors studied is the glycosaminoglycan heparin, which inhibits VSMC proliferation and migration both in cell culture and in animal models (Mishra-Gorur K, Delmolino LM, Castellot Jr JJ: Biological functions of heparan sulfate and heparan sulfate proteoglycans. Trends Glycosci Glycotechnol 1998, 10:193-210). To aid our understanding of the anti-proliferative mechanism of action of heparin, we used a subtractive hybridization approach to isolate and characterize a novel growth arrest-specific (gas) gene induced in VSMCs exposed to heparin (Delmolino LM, Stearns NA, Castellot Jr JJ: Heparin induces a member of the CCN family which has characteristics of a growth arrest specific gene. Mol Biol Cell 1997, 8:287a and Delmolino LM, Stearns NA, Castellot Jr JJ: COP-1, a member of the CCN family, is a heparin-induced growth arrest specific gene in vascular smooth muscle cells. J Cell Physiol 2001, 188:45-55). This gene is a member of the cysteine-rich 61/connective tissue growth factor/nephroblastoma-overexpressed (CCN) family and has been given the name CCN5. In this report, we provide functional evidence that CCN5 can inhibit VSMC proliferation, motility, and invasiveness. In contrast, adhesion and apoptosis are unaffected by CCN5 in this cell type. We also significantly extend previous data from our laboratory that suggests CCN5 is a growth arrest-specific (gas) gene. Furthermore, we map for the first time the cellular localization of CCN5 protein in cultured VSMCs. We also examine uninjured and balloon-injured rat carotid arteries for CCN5 expression. The results from the in vitro and in vivo localization studies show that CCN5 is temporally and spatially expressed in a manner consistent with a role in regulating proliferation, motility, and invasiveness of VSMCs.