Acute lymphoblastic leukaemia (ALL) involving translocations of the Mixed Lineage Leukaemia (MLL) oncogene accounts for over 60% of infant ALL. These so-called infant MLLs exhibit unique clinical and biological features associated with poor treatment outcome. This study aimed to identify novel drug targets in infant MLL by carrying out an unbiased kinome-wide analysis of tyrosine phosphorylation, using an antibody-based phosphotyrosine enrichment coupled with mass spectrometry (MS), across a panel of 8 infant MLL xenografts established from patient explants in immune-deficient mice. Consistent with published data xenografts exhibited high expression of phosphorylated FLT-3. Additional highly tyrosine phosphorylated proteins included non-receptor tyrosine kinases (NRTKs) such as the Src family kinases Lyn, Fyn and Fgr, the Janus kinase JAK-2, Syk and Fes, and the RTK Axl. Downstream effectors including Gab, Stat-5 and Erk-1/2 were also highly phosphorylated, reflecting activation of key pathways involved in cell proliferation/survival. MS data were confirmed by immunoblotting, and interrogation of intracellular protein-protein interactions by immunoprecipitation/immunoblotting. Axl was found to interact with JAK-2, Fgr, Lyn and Fyn, consistent with its role as a docking site for NRTKs. Ex vivo exposure of xenograft cells to the Axl inhibitor R428 for 48h resulted in IC50 values of <1µM, assessed by Alamar blue cytotoxicity assay. Furthermore, exposure of two xenografts with particularly high phospho-Axl expression (MLL-1 and MLL-5) to 1µM R428 resulted in rapid and profound decrease in Axl protein expression (to <30% of control by 8h). Decreased Axl expression was accompanied by marked down-regulation of Fes and JAK-2 expression in both xenografts. Finally, R428 and the JAK-2/3 inhibitor TG101209 exerted synergistic cytotoxicity against MLL-5 cells ex vivo. These results suggest that Axl is overexpressed and activated in a sub-population of infant MLLs resulting in stabilisation and activation of NRTKs, and that specific targeting of Axl may represent a novel therapeutic approach.