Rap GTPases are Ras-family GTPases and play crucial roles in regulating cell adhesion/polarity and gene activation in a variety of cell contexts (Adv. Immunol. 2007). Rap signaling is essential for normal lymphohematopoiesis of all lineages, including myeloid, B- and T-lineage cells (Blood 2008a, 2009). Spa-1 (Sipa1) is a principal Rap-specific GTPase-activating protein (GAP) (JBC 1999, J. Biochemist. 2003), and deregulated Rap activation by Spa-1-defciency results in a spectrum of lymphohematopoietic malignancies due in part to enhanced c-Myc expression with increased genomic instability in hematopoietic progenitors (Cancer Cell 2003, Immunity 2006, Cancer Res. 2008, Cancer Sci. 2010). Recent analysis also reveals that excessive Rap signaling due to overexpression of Rap GTP/GDP exchange factor (C3G) in Spa-1-defciency causes aberrant homing of hematopoietic stem cells (HSCs) on BM transplantation. Raphigh HSCs preferentially home to thymus and spleen rather than BM unlike normal HSCs. Most notably, the Raphigh HSCs develop Notch-dependent T-cell acute lymphoblastic leukemia (T-ALL) with frequent Notch mutations, highly reminiscent of human T-ALL (Blood 2008b). The T-ALL genesis absolutely requires thymic microenvironment, in that Raphigh HSCs transplanted in athymic nude mice show extramedullary hematopoiesis in the spleen but never develop T-ALL. Rap signaling is crucial for the continuous Notch processing/activation and therefore maintenance of leukemic potential of T-ALL cells. In this presentation, I shall focus on the mechanisms of Notch activation by Rap signaling and provide a novel molecular target for controlling human T-ALL. Speaker affiliation: Kyoto University Graduate School of Medicine, JP