In agreement with this magic size, retroviral expression of present in over 50% of T-ALL patients at diagnosis 21 has brought enormous interest for the development of molecularly personalized therapies in T-ALL and prompted the initiation of a clinical trial to test the effectiveness of blocking NOTCH1 signaling with a small molecule GSI with this disease. T-ALL. in hematopoietic progenitors fail to develop T-cells, and display ectopic B-cell development in the thymus. 11 In addition to this early part in T-cell lineage commitment, Clioquinol NOTCH signaling participates, later on, in essential processes at multiple phases of thymocyte development. 14 Therefore, NOTCH1 activity is required for lineage progression through the early Clioquinol DN1, DN2 and DN3 phases of thymocyte development; 15 participates in the rules of T-cell receptor loci; 16 and regulates lineage decisions between vs. lineages. 17 The first Clioquinol evidence linking aberrant NOTCH1 signaling to the pathogenesis of T-ALL came from the characterization of the t(7;9)(q34;q34.3) translocation, a rare recurrent chromosomal rearrangement present in less than 1% of human being T-ALL instances. This translocation juxtaposes a truncated NOTCH1 gene alongside the T-cell receptor B (in the pathogenesis of human being T-ALL was not fully realized until the recognition of activating mutations in in the majority of T-ALL patient samples.21 Activating mutations in NOTCH1 result in ligand independent activation of the receptor or improved ICN1 protein stability in over 50% of T-ALLs.21, 22 Clioquinol 23 24, 25 Importantly, small molecule inhibitors of the -secretase complex (GSIs) effectively abrogate the function of the receptor encoded by these oncogenic alleles, making NOTCH1 a promising therapeutic target for the treatment of T-ALL. Initial studies treating human being T-ALL cell lines harboring activating mutations in with GSIs shown that inhibition of NOTCH signaling induces cell cycle arrest.21, 26, 27 However, these same studies also demonstrated that despite high prevalence of mutations and the presence of high levels of ICN1 in these tumors, the majority of T-ALL cell lines failed to respond to NOTCH1 inhibition, suggesting that main resistance to GSI treatment was readily present in the majority of these cell lines.21 Numerous mechanisms have been identified in the resistance of cancer cells to chemotherapy and molecularly targeted medicines. 28-30 General mechanisms of drug resistance typically decrease the effective intracellular concentration of the drug due to decreased drug uptake, improved drug export or drug rate of metabolism. In the case of molecularly targeted medicines, resistance typically results from mutations that block the interaction of the drug with its specific molecular target. A common feature in all these mechanisms is that defective cellular responses are the result of prolonged biochemical activity of the molecules and pathways that mediate drug response. Therefore, the initial experiments aimed to identify the mechanisms of resistance to GSIs focused on the analysis of NOTCH1 activity. Remarkably, these studies exposed that GSI treatment efficiently clogged -secretase activity and as a result, induced decreased levels of ICN1 and transcriptional downregulation of NOTCH1 target genes in GSI-resistant T-ALL cell lines.31 This observation indicated that the lack of response to GSIs happens despite effective NOTCH1 inhibition, and pointed to the existence of alternative pathways promoting cell growth in the absence of NOTCH1 signaling in these tumors. Therefore, we postulated that understanding of the mechanisms of level of VHL sensitivity and resistance to GSIs in T-ALL required the recognition of the key downstream target genes and pathways responsible for NOTCH1 induced transformation. Microarray gene manifestation studies and ChIP-on-chip analysis have recently recognized a major part of NOTCH1 in the rules of cell growth and rate of metabolism.32 Thus, NOTCH1 inhibition having a GSI drives a gene manifestation signature dominated from the downregulation.