Hemangioblasts in adult bone marrow participate in new blood vessel formation. By modulating the differentiation of hemangioblasts into blood vessel cells, angiogenesis in a particular tissue can be increased or decreased. The present invention features compositions and methods for reducing tumor vasculogenesis, treating leukemia, and/or treating or preventing leukemia relapse. In particular, the invention provides an SDF-1 binding agent (e.g., antibody, antisense, ribozyme) for the treatment or prevention of a neoplasia, such as leukemia. Intravitreal injection of antibodies that block SDF-1 activity inhibited induced retinal neovascularization mediated by hemangioblasts. Anti-SDF-1 ribozymes and SDF-1 anti-sense RNA expression constructs significantly reduced migration of cells that create new vessels in the eye.