Steady state levels of transcription factors play an important role in proliferation and differentiation of hematopoietic cells. The transcription factor c-Myb is frequently activated by retrovirus integration in murine and avian leukemias. Its deregulation has been also implicated in human acute and chronic leukemias and some other nonhematopoietic tumors. It is a short-lived protein, which is rapidly degraded by the 26S proteasome. Truncation at the carboxyl (COOH) terminus, which has occurred in some oncogenic forms ofc-Myb, results in the increased resistance to proteolysis. This stabilization correlates in vitro with less efficient ubiquitination. Here, we report the first evidence of post-translational modification of c-Myb by ubiquitin in vivo using HA-labeled ubiquitin. We also show that, in contrast to the unstable wild type or amino (NH2)-terminally truncated c-Myb form, stable carboxyl (COOH)-terminally truncated c-Myb is not targeted to degradation by covalent attachment of ubiquitin in vivo. In addition, following an analysis of subcellular fractionation of proteins from cells treated with a 26S proteasome inhibitor we were able to localize c-Myb exclusively in the nuclear compartment, suggesting the absence of a requirement for export to cytoplasm prior proteolytic processing. Furthermore, pulse-chase experiments of c-Myb protein isolated from interphase cells or cells synchronized in the G2/M or G1 phases of cell cycle did not reveal substantial cell cycle dependent differences in proteolytic processing by the 26S proteasome. Also, the demonstration that the half-life of c-Myb in myeloid progenitor M1 cells induced to differentiate along the monocytic pathway is the same as in undifferentiated cells suggested that proteolytic breakdown of c-Myb is a constitutive process during proliferation and differentiation.