The MRD concept. x-axis represents time; y-axis represents tumor burden. The horizontal black line represents the sensitivity of standard morphologic analyses, such as light microscopy using immunohistochemical methods. I, II, and III indicate 3 different courses of disease documented by MRD measurements. Patient course I (early relapse) can be distinguished from II and III (late relapse and complete cytogenetic response, respectively) by MRD measurements during therapy (period A). Relapse (patient course I and II) can in some cases be identified several months before clinical symptoms by MRD measurements after discontinuation of therapy (period B).