Eukaryotic organisms have developed a complex network of cell cycle control pathways in an attempt to ensure the faithful replication and division of the genome. One important component of this network is the cyclin- dependent kinase inhibitor p21 which is upregulated in normal cells in response to a number of cellular stresses, including ionizing radiation (IR). However, we have discovered that, in transformed cells, ionizing radiation leads to the degradation of p21. We show that despite being induced by IR, this process is independent of the most well characterized transducers of the DNA damage signal, the PI3-like family of protein kinases (PIKK). We demonstrate degradation of p21 after IR is ubiquitin- dependent and requires the Cul4-DDB1Cdt2 E3 ligase. In addition, we show that mutations in p21 that prevent it from binding to PCNA also prevent its degradation after IR. As p21 degradation has been implicated in facilitating DNA repair, the specificity of this event for transformed cells suggests that these cells may harbor an additional DNA damage repair pathway not present in normal cells. In a separate project, we have worked to establish a mouse model of Chronic Myelogenous Leukemia (CML). CML is a chronic myeloproliferative disorder which inevitably progresses to a terminal blast crisis phase without treatment. In human patients, blast crisis is characterized by the presence of abnormal granulocyte macrophage progenitors (GMP) that have increased self- renewal capacity and elevated levels of nuclear beta- catenin. We have found that expression of p210BCR-ABL in an established line of pluripotent E2A-knockout mouse bone marrow cells leads to expansion of a leukemia-initiating GMP-like population of cells. Upon transplantation into mice, these cells generate a CML-like disease that can be transplanted into secondary recipients. Analysis of the leukemic GMP demonstrated that these cells have higher levels of beta-catenin activity, both in vitro and in vivo, than either the transformed non-GMP or the untransformed GMP. These data provide additional evidence that GMP-like cells with elevated levels of beta-catenin activity may function as the leukemic stem cells in CML