A number of leukemia-specific chromosomal translocations have been identified that have been cloned and are appropriate markers for molecular studies (Table 1 ) (1 –4 ). In addition, leukemia nonspecific clonality markers, such as the junctional region of the rearranged immunoglobulin (Ig) and T-cell receptor (TCR) genes can be used for minimal residual disease studies. It is commonly accepted that the Ig heavy chain (IgH) gene junctional regions as well as the junctional regions of rearranged TCR-μ and TCR-δ can be used as targets for polymerase chain reaction (PCR) analysis (1 ,2 ). The detection and quantification of leukemia specific rearrangements will be explained on the example chronic myelogenous leukemia (CML), since this disease was the first human tumor associated with a specific chromosomal rearrangement and a specific fusion gene. The spectrum of molecular methods to detect fusion genes and their products has been established on CML during the last 15 years.
