Continuous MLL-ENL expression is necessary to establish a "Hox Code" and maintain immortalization of hematopoietic progenitor cells..     ///    "Thus, MLL-ENL is required to initiate and maintain immortalization of myeloid progenitors and may contribute to leukemogenesis by aberrantly sustaining the expression of a ""Hox code"" consisting of Hoxa4 to Hoxa11.. "    ///    "By comparing these changes with Hox gene expression in cells induced to differentiate with granulocyte colony-stimulating factor, we show for the first time that reduced Hox gene expression is specific to loss of MLL-ENL and is not a consequence of differentiation. "    ///    To investigate the molecular mechanism of immortalization by MLL-ENL we established a Tet-regulatable system of MLL-ENL expression in primary hematopoietic progenitor cells.     ///    This analysis showed that loss of MLL-ENL was accompanied by a reduction in the expression of multiple Hoxa genes.     ///    These cells either terminally differentiate or die when MLL-ENL expression is turned off with doxycycline.     ///    "Immortalized myeloid cell lines were generated, which are dependent on continued MLL-ENL expression for their survival and proliferation. "