MLL is a gene involved in translocations associated with leukemia. It encodes a protein with PHD fingers and SET domains that are conserved in the related Drosophila protein trithorax. Cyp33, a nuclear cyclophilin with an RNA binding domain, interacts with the MLL PHD finger domain. Overexpression of Cyp33 represses the Hoxc8 gene in human leukemia cells in an MLL dependent manner. This close relationship between Cyp33 and the genes involved in hematopoiesis and leukemogenesis inspires us to further clarify the function of Cyp33 in primary hematopoietic cells. Murine bone marrow progenitor cells (BMPC) were retrovirally transduced to express Cyp33 and their replicative potential was then investigated by repeatedly plating on methylcellulose medium supplemented with GM-CSF. Upon over-expression of Cyp33, p27 is down-regulated and the progenitor cells keep proliferating after the fourth plating while the control cells (infected with empty retroviral vector only) differentiate and stop forming colonies after the third plating. Furthermore, after the third plating, Cyp33 over-expressing cells exhibit primitive cell morphology after Wright staining, while the control cells show mature macrophage/granulocyte characteristics. We also generated a Cyp33-BMPC line by growing the Cyp33 over-expressing cells in liquid culture containing GM-CSF, after the fourth plating. Regardless of the presence of GM-CSF, the Cyp33-BMPC cell line cycles and grows exponentially maintaining stem cell surface markers: Sca-1+, c-Kit+, CD34+ and Lin− (B220−, CD3e−, Mac-1−, Gr-1−). Morphologically, Cyp33-BMPCs are like immature progenitor cells. In addition, these cells can again form compact colonies on the same methylcellulose media as described before. These data suggest that Cyp33 may immortalize mouse bone marrow progenitor cells and arrest them at the primitive cell stage.