Fyn is 59-kDa member of the Src family of kinases that is historically associated with T-cell and neuronal signaling in development and normal cellular physiology. Whereas Src has been heavily studied in cancer, less attention has been traditionally awarded to the other Src kinases such as Fyn. Our group has shown that Fyn is particularly upregulated in prostate cancer in contrast to the alternative members of the Src family. This suggests that it may mediate several important processes attributed to Src kinases in prostate cancer and other malignancies. These functions include not only cellular growth and proliferation but also morphogenesis and cellular motility. Together, these suggest a role for Fyn in both progression and metastasis. As several agents in clinical development affect Fyn activation, understanding the role that Fyn plays in cancer is of great importance in oncology. Cancer 2010. (c) 2010 American Cancer Society.