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Molecular genetics is a powerful new tool for detection, diagnosis, evaluation of treatment, and understanding of the biology of malignant disease. The first malignant disease to be characterized by a specific cytogenetic aneuploid abnormality was chronic myelogenous leukemia (CML) which was first recognized in 1962 as an abnormal shortening of the long arm of either chromosome 21 or 22, the smallest chromosomes in the human genome. This cytogenetic abnormality was subsequently shown to represent not a loss of genetic material, but a pseudodiploid aneuploidy with a reciprocal translocation of chromosomal material from the long arms of chromosome 9 and 22. The discovery that the abl oncogene was located at the breakpoint on chromosome 9 allowed the molecular geneticists to identify the genetic material adjacent to the translocated material from chromosome 9 and because the breakpoint on chromosome 22 was variable but restricted to a small region, it was named the breakpoint cluster region (BCR). The BCR/abl gene has been found exclusively in leukemic cells from patients with CML. The somatic cells of the host are not involved and at least to date, every individual that has been found to have this gene has also had the disease CML. Approximately 15 % of CML patients lack the Philadelphia chromosome on cytogenetics. However, approximately half of these patients can be shown to have the BCR/abl neogene present as a masked translocation.

We have a broad spectrum of treatment techniques which can render patients with diagnosable malignancy free of disease (N.E.D. or C.R). Such patients depending on the characteristics of the original tumor are at variable risks of recurrence of disease. It is clear that patients in complete remission can benefit from adjuvant treatment given during the disease free period. The basis for such adjuvant therapy depends on the probability that the patient will develop recurrent disease. The prescription of adjuvant treatment results in offering treatment to some patients who are already cured of their disease in an effort to prolong the disease free period for patients who have residual disease. Thus, a pressing problem in the field of adjuvant therapy is the development of techniques for the objective detection of residual disease which would serve as a guide to the treating physician and would maximize the benefit/risk ratios for each individual patient.