Atherosclerosis is the leading cause of cardiovascular disease, the highest worldwide death accountee. It is mainly influenced by genetical and environmental factors. In atherosclerosis, smooth muscle cells (SMCs) play an important role. Our lab has identified two distinct phenotypes of SMCs isolated in vitro from pig coronary arteries: spindle-shaped SMCs (S-SMCs) and rhomboid (R-SMCs). R-SMCs acquire a synthetic phenotype during atherosclerosis and specifically express S100A4 in pig SMCs in vitro and human samples. The aim of this master thesis project was to decipher the role of intracellular S100A4 in the phenotypic switch of SMCs. By using the S100A4 KO mouse model we isolated and compared WT and KO SMCs in vitro using immunohistochemistry, western blot and migration assay experiment. We studied markers and pathways we know to be affected in SMCs during atherosclerosis, such as proliferation, migration, and inflammation. IFs done for a-SMA and S100A4 showed an increase of intracellular S100A4 (in WT cells) when cells are treated with PDGF-BB and S100A4 alone or in combination. We saw a decrease of a-SMA as well but only in PDGF-BB treated cells. Those results were confirmed by western blot and expected regarding previous experiments done on pig SMCs. Western blot analysis showed a higher amount of myosin in KO cells compared to WT. Migration assay with Boyden chamber showed a lack of migratory activity for S100A4 KO SMCs for all conditions which could be related to myosin disregulation. In conclusion, during my thesis, we show that Intracellular S100A4 expression in SMCs seems to have an impact on their migration capacity which they need to move from the media to the intima during early atherosclerosis development. This deficiency might be caused by an impairement of the interaction between S100A proteins family and myosin filaments which they regulate its disassembly. Further experiments should be done in order to confirm this hypothesis.