Methods and compositions are provided for the identification of stem cells and cancer stem cells. .beta.-catenin is also identified as a target for the development of therapeutic moieties against hematopoietic tumors, i.e. leukemia and lymphoma cells, which may include screening assays directed at .beta.-catenin, or members of the .beta.-catenin signaling pathway. Cellular proliferation in hematopoietic cells can be altered by introducing stabilized .beta.-catenin into a hematopoietic cell that is altered in its ability to undergo apoptosis but which is not fully transformed. The immortalized cells are useful in screening assays, and in the analysis of pathways by which hematopoietic cells undergo transformation.