Genomic instabilities including chromosomal translocations are frequently associated with genetic diseases and cancer, especially leukemia. Cytogenetic studies of these diseases requiring preparation of metaphase chromosomes are often key to revealing their chromosomal abnormalities. Treatment of in vitro disease cell cultures with cell cycle inhibitors, e.g. colcemid, have proved to be a very simple and effective metaphase preparation method for cytogenetic studies. Two good examples are metaphase preparation from cultured cells and some types of myeloid leukemia. However, this approach has some limitions and disadvantages: 1) it is difficult to grow many types of primary cell in vitro; 2) in vitro cultures tend to introduce additional artificial mutations to the genome. To overcome these issues, we have established an approach to prepare metaphase chromosomes directly from murine bone marrow. With this new approach, we were able to get 5-10% of metaphase chromosomes from primary bone marrow leukocytes and identify chromosomal translocations recurring in a Pten null murine model developing T cell leukemia (Figure 1,2 [1]).