Soluble factors such as growth factors and cytokines present in the tumor microenvironment regulate a variety of genes associated with malignant properties of tumor cells such as growth, migration, invasion, and metastatic capacities. CD44 is a multi-functional adhesion molecule involved in cell to cell and cell to extracellular matrix interaction, the trapping of growth factors and cytokines, and the regulation of cell traffic. Growth factors and cytokines modify the expression, selective isoform splicing and functions of CD44, resulting in changes in the biological properties of the cells. These include adhesion of circulating tumor cells to endothelium and body cavities, and survival in response to growth factors presented by the CD44 molecule. The modification of CD44 on both tumor and host cells by growth factors may play an important role in tumor progression.