Mixed lineage leukaemia (MLL) mutation accounts for 60-80% of all infant acute leukaemia’s and is an aggressive tumour that needs high amounts of poly-chemotherapy that often result in a poor prognosis outcome. Genetically, paediatric and adult acute leukaemia patients are characterized by distinct genetic rearrangements of the MLL gene, located at the 11q23 chromosomal band. MLL has been found in 73 different translocations and 54 partner genes have been cloned. Only 6 frequent partner proteins constitute 85% of all clinical cases of MLL, one of which is mixed lineage leukemia-AF9 (MLL-AF9). Fusion proteins (FPs) are generally transcriptional activators that induce ectopic expression of target genes in hematopoietic precursor cells. MLL-FP’s are capable of leukemic transformation and the dysregulation of multiple genes, often through the aberrant recruitment of epigenetic modifying enzymes such as histone deacetylases and methyltransferases as well as deregulated cell cycle control. Using retroviral gene transduction of hematopoietic stem cells we have produced mice that develop acute leukemia driven by MLL-AF9 +/- the oncogenic driver NRAS. These mice were utilized to study the oncogenic potential and key molecular targets for novel therapies of these tumours. Given that MLLAF9 recruits methyltransferase protein Dot1L that in turn recruits pTEFb a dimmer of CDK9, thus increasing phosphorylated RNA Pol II, CDK9 inhibition is a rational therapeutic approach in this model. In collaboration with Merck, Dinaciclib (SCH 727965) has proven to be a superior pan cyclin-dependent inhibitor with low nanomolar potency against CDK9. We have found that this compound is highly cytotoxic to the leukemic cells in comparison to conventional chemotherapies and recently published therapies for this disease. The compound shows effective anti-proliferative activity in tumours with a GI50 of 9nM and is capable of inducing apoptosis as determined by Annexin V/PI double staining, at 24h with IC50 of ~30nM.