The focus of the study of minimal residual disease (MRD) is to redefine the concept of remission by using more sensitive molecular techniques to detect level of disease burden below that of conventional pathology. The detection of the chimeric bcr-abl mRNA transcript in chronic myeloid leukemia (CML) is a paradigm of the use of molecular biology for clinical applications. The qualitative (yes vs no) detection of MRD is associated with a relative increase in relapse rate, and the magnitude of the relative risk appears dependent on the time from transplant, and the type of transplant. The quantification of disease burden by quantitative PCR (Q-PCR) can greatly strengthen the relationship of MRD and subsequent relapse. In addition, the promise of genomics offers hope that in the near future, leukemia may be sub-classified by the genetic profile of an individual patient’s particular leukemia, allowing truly [quot ]tailored[quot ] individual therapy.