CXCL12 and leukemia: We used a cytokine concentration of 100 ng/ml, as it has previously been used in similar screens.16 As a proof of principle, the screen identified several cytokines as selective positive regulators of leukemia cells previously associated with leukemic stem and progenitor cell biology, including CXCL12a (SDF1), IL3, tumor necrosis factor α and IL6 (Figure 1b).6, 7, 17, 18 In addition, several selective negative regulators of AML cells were identified and, of these, IL4 elicited the strongest depletion of leukemia cells (Figure 1b).