Inactivation of the NF1 tumor suppressor causes myeloproliferative diseases. NF1 encodes a GTPase activating protein (GAP) for Ras. Myeloid cells with loss of NF1 have high levels of Ras-GTP, functionally equivalent to the effects of RAS oncogenes. We investigated the effects of the NF1 GAP-related domain (GRD) in proliferation, apoptosis and Ras-GTP levels in Nf1-negative acute myeloid leukemia (AML) cells. In AML cells, with cooperating mutations, the expression of the neurofibromin GRD causes significant reductions of N- and K-Ras-GTP levels, which is not incompatible with AML cell survival, but which is strongly selected against due to suppression of proliferation.