Deregulated RTK activity has been implicated as a causal leukemogenic factor in the context of molecular aberrations that perturb differentiation in the hematopoietic lineage such as in childhood ALL. A deeper understanding of RTK signaling processes on a system-wide scale will be key in defining critical components of signaling networks. To link RTK activity with in vivo output in primary ALL we took a functional approach, which combined SH2 domain binding, mass spectrometry, and transcriptome analyses. Structure and composition of evolving networks were highly diverse with few generic features determined by receptor and cell type. A combinatorial assembly of varying context-dependent and few generic signaling components at multiple levels likely generates output specificity. PAK2 was identified as a phosphoregulated FLT3 target, whose allosteric inhibition resulted in apoptosis of ALL cells. Our studies provide evidence that a functional approach to leukemia signaling may yield valuable information for a network-directed intervention.