Transcription is a tightly regulated process ensuring the proper expression of numerous genes regulating all aspects of cellular behavior. cells enable the generation of pre-leukemic cell models for the analysis of aberrant human hematopoietic progenitor cell expansion mediated by leukemogenic transcription factors. This review summarizes recent findings regarding the mechanism by which leukemogenic gene products control human hematopoietic CD34+ progenitor cell expansion by disrupting the normal epigenetic program. long-term expansion using an alternative approach leukemia-associated oncogenes can be delivered and stably expressed by the retroviral gene transfer technology. The most commonly used delivery system is the gamma-retroviral vector system based on the Moloney murine leukemia virus (Mo-MLV) genome (Kohn et al. 1987 Cd8a The murine stem cell virus (MSCV) expression vector is one of the most frequently employed gamma-retroviral vector systems as it enables stable and high transgene expression in virtually all cell types (Hawley et al. R788 1994 Lentiviral vectors which are based on the HIV genome display an increased capacity to incorporate large transgenes (up to 10 kilobases); although vector titers decrease when using larger inserts (Matrai et al. 2010 Lentiviral transduction efficacy can be further improved by concentrating the viral particles via ultracentrifugation (Naldini et al. 1996 Kanbe and Zhang 2004 RetroNectin-based gene transduction protocols dramatically enhance the efficiency of retrovirus-mediated gene transfer in hematopoietic suspension cells. With this system retroviral particles are preloaded onto RetroNectin-coated surfaces and co-localize viral particles and target cells into close proximity thereby markedly increasing the transduction efficiency (Hanenberg et al. 1996 Expression of a gene R788 of interest is usually coupled to the expression of a marker gene e.g. enhanced green fluorescent protein (eGFP) which allows for the immediate determination of viral transduction efficacy and the identification of transduced cells to assess proliferation differentiation and cell death (Figures 1C-F). Leukemic transcription factors epigenetically control progenitor cell expansion In principle the term epigenetic regulation refers to any stable mitotically perpetuated regulatory mechanism of a genome that does not alter the primary nucleotide sequence (Jaenisch and Bird 2003 Oki and Issa 2010 DNA methylation histone modification histone variant deposition in gene bodies R788 and recruitment of transcription-related enzymes to specific genetic loci are the most commonly known molecular mechanisms that mediate epigenetic phenomena. DNA methyltransferases (DNMTs) are the key enzymes of genome methylation which play an important role in the epigenetic regulation of gene expression and repression (Jackson-Grusby et al. 2001 Jaenisch and Bird 2003 In general DNMT1 maintains DNA methylation in mammalian cells while DNMT3A and DNMT3B act as DNMTs by methylating unmethylated CpG sites (Oki and Issa 2010 Recent studies have demonstrated that DNA methylation is critical for the self-renewal and differentiation of normal and leukemic stem cells (Hogart et al. 2012 Moreover posttranslational modification of histones regulates chromatin structure and transcription. Histone acetylation and methylation alter gene expression patterns and cellular behavior during the onset and progression of oncogenesis (Ellis et al. 2009 Furthermore repressive histone modification mediated by Polycomb-group (PcG) complexes is involved in the balance between the self-renewal and differentiation of hematopoietic stem cells via regulation of the cell cycle. PcG proteins are histone modifiers found in two R788 protein complexes Polycomb Repressive Complex (PRC) 1 and PRC2 which target cis-regulatory polycomb response elements (PREs) by normal and aberrant transcription factors (Cedar and Bergman 2009 PRC2 the “initiating complex” catalyzes the di- and tri-methylation of histone H3 at lysine 27 (H3K27me3) accompanied by the gene repression and maintenance of self-renewal programs of leukemic stem cells (Sashida R788 and R788 Iwama 2012 Following PRC2-mediated histone methylation the PRC1 complex (“maintenacnce complex”) is recruited to chromatin via binding to H3K27me3. Forced expression of PcG genes such as BMI1 or EZH2 enhances the self-renewal capacity of HSCs and obviates long-term repopulating exhaustion during serial transplantation (Iwama et al..