The transcription factor v-Myb is a potent inducer of myeloid leukemias, and its cellular homologue c-Myb plays a crucial role in the regulation of hematopoiesis. Recently, Bies and coworkers (Bies, J., Markus, J. & Wolff, L. (2002)J. Biol. Chem, 277, 8999–9009) presented evidence that murine c-Myb can be sumoylated under overexpression conditions in COS7 cells when cotrans-fected with FLAG-tagged SUMO-1.