In a leukemic bone marrow, the deregulation of cytokines contributes to an alteration in bone marrow niche architecture, which contributes to leukemic progression, and suppression of normal hematopoiesis.3, 27, 28, 29, 30 We here established a cytokine screen using murine fluorescently labeled MLL-AF9 AML cells and identified IL4 as a selective negative regulator of primitive AML cells. The gene discussed is IL4; the disease is acute myeloid leukemia.