The Philadelphia chromosome (Ph1) was the first genetic change to be associated consistently with leukemia, and it is one of the best understood on the molecular level. Because of this, it is an excellent model to investigate the application of molecular techniques to the clinical setting. These techniques are reviewed as are their clinical use in chronic myelogenous leukemia (CML), acute lymphoblastic leukemia (ALL), and transplantation. The Ph1 is caused by the fusion of two genes on chromosomes 9 and 22, resulting in the BCR-ABL fusion gene. This new gene is believed to be the cause of these Ph1-positive leukemias. The ability to detect the BCR-ABL fusion gene evolved from cytogenetic detection to Southern blot analysis, and now includes sophisticated techniques such as polymerase chain reaction (PCR) methods and pulsed-field gels. Diagnosis of the BCR-ABL fusion gene by Southern blot detection of bcr genetic rearrangements is the prototype of molecular cancer diagnosis. The sensitivity and clinical uses of this test are reviewed, especially its application to monitoring the response to treatment. PCR methods enable the researcher to detect 1 CML cell in a population of 10(5) cells. Clinical experience with PCR, especially in transplantation medicine, is providing a better understanding of the meaning of the terms "remission" and "cure." Newer techniques using fluorescent in situ hybridization have considerable potential for BCR-ABL detection, but no clinical experience has been gained with these techniques currently. The diagnosis of the BCR-ABL fusion gene in ALL has important clinical implications because it is the most common molecular genetic change in adult ALL and is associated with short remissions and poor outcome in all age groups. Diagnosis of the BCR-ABL fusion in ALL is difficult because the molecular findings are more heterogeneous than they are in CML. The methods available and their accuracy and sensitivity are compared. A review of their clinical impact is included.