S100A4 (also known as Mts1, metastasin, p9Ka, pEL98, CAPL, calvasculin, Fsp-1, 
placental calcium-binding protein) belongs to the family of EF-hand 
calcium-binding proteins, whose expression is elevated in a number of 
pathological conditions. Although it is well documented that S100A4 is expressed 
in cancer cells and contributes to tumor cell motility and metastatic 
progression, the exact underlying mechanisms remain elusive. An important 
characteristic feature of S100 proteins is their dual function, inside and 
outside the cell. In this review, we focus on the intracellular function of 
S100A4. The review contains structural analysis of S1004 in comparison with 
other members of S100 proteins. Possible modes of the interaction of S100 
proteins with targets are described. Several examples of best-studied molecular 
interactions involving S100A4 with heavy chain of nonmuscle myosin IIA, 
LAR-interacting protein liprin beta1 and tumor suppressor protein p53 are 
provided. We suggest that the binding of S100A4 to these molecules is critical 
for the S100A4 function. Further studies of the implications of these 
interactions in different molecular pathways may shed additional light on the 
role of S100A4 protein in the control of tumor cell motility and migration. We 
discuss the approaches for down-regulation of S100A4 expression and their 
potential for application in the clinics.