IntroductionThe recent success of monoclonal antibodies and small-molecule inhibitors of tyrosine kinases in numerous malignancies have highlighted the potential of targeted therapy for the treatment of cancer. [1][2][3][4] However, broad application of this strategy will require a more detailed understanding of the principal genetic targets involved in cancer pathogenesis in each individual patient. Tyrosine kinases constitute a gene family of 91 members that have an integral role in signal transduction of mammalian cells, including critical cellular processes as diverse as proliferation, apoptosis, differentiation, and cell motililty. Aberrant regulation of any of these processes might contribute to oncogenesis, thus it is not surprising that dysregulation of tyrosine kinase activity has been observed in numerous types of malignancy. 5 Acute myeloid leukemia (AML) represents one malignancy in which tyrosine kinases are abnormally regulated. Previous studies have shown that phosphorylation of signal transducer and activator of transcription 5 (STAT5) is present in blast cells from at least 70% of patients with AML. [6][7][8] Because STAT5 phosphorylation is tightly controlled by tyrosine kinase signaling networks, this suggests the presence of constitutively active, mutated tyrosine kinases in these patients. To date, the only known activating mutations in tyrosine kinases in AML are point mutations in c-KIT (5%), mutations or internal tandem duplications in FLT3 (30%), and rare mutations observed in JAK2, JAK3, and PDGFR (Figure 1). [9][10][11][12][13][14][15][16][17][18][19] These known abnormalities in tyrosine kinases give mechanistic insight into the genetics underlying approximately half of the cases of AML with phospho-STAT5. Of the remaining cases with unknown genetic etiology, the presence of phosphorylated STAT5 suggests that the tyrosine kinase family is one likely source of unknown oncogenic mutations (Figure 1).To determine the identity of novel mutant genes in cancer, numerous approaches have been used. One strategy involves large-scale sequencing of selected or entire cancer genomes. While this process has uncovered numerous mutations, the functional role of many of these genetic abnormalities remains unclear. 20,21 We have previously reported an alternate strategy that employs phosphoproteomic profiling of cells as a means of guiding sequencing studies to likely sources of mutations. 17,[22][23][24] Despite the successes of both of these approaches, an alternative strategy that directly delivers functional information about important genes could offer even greater diagnostic potential for AML as well as other malignancies. RNAi technology allows functional data to be obtained by selectively reducing the expression of individual genes, thus allowing the necessity of those genes for cancer cell viability to be assessed. 25,26 Indeed, numerous studies have used individual and multiplexed RNAi screens to better understand radiation and DNA damage susceptibility, mitotic progression, angiogenesis, tu...

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