Cytogenetic methods have become increasingly important tools for both research in hematological malignancies and for the diagnostic workup of leukemias and lymphomas. The knowledge about specific chromosomal aberrations has been an essential prerequisite for the identification of pathogenetically relevant genes. Important examples are molecular genetic analyses of the breakpoint regions in chromosomal translocations, which resulted in the detection of protooncogenes such as ABL in chronic myeloid leukemia (CML) and acute lymphoblastic leukemia (ALL) [t(9,22)(q34;q11)], or MYC in Burkitt's lymphoma [t(8;14)(q24;q32); for a review see refs. 1 and 2].