The past decade has brought new technologies to the study of minimal residual disease (MRD) in leukemia. Each of them has limitations and is far from being accurate. Recently, a new multiparametric cell scanning system (Duet) was introduced to the field of MRD detection. This system has the advantage of automatically scanning large numbers of cells and performing combined analysis of morphology and fluorescence in situ hybridization (FISH) on the same cell. We used this system to characterize the lineage and degree of maturation of the cells carrying the minor m-BCR/ABL fusion, in a follow-up of an 8-year-old boy with Philadelphia-positive (Ph(+)) acute lymphoblastic leukemia (ALL). The boy was treated using a high-risk protocol and was closely monitored with FISH analysis for cells carrying the m-BCR/ABL fusion. Consecutive analysis along 2.5 years from remission showed 0.2-4.5% m-BCR/ALB(+) cells in the peripheral blood (PB), which is within the accepted background range for this method. The combined analysis found that all the m-BCR/ABL(+) cells were mature lymphocytes. Because mature lymphocytes have a long life span in the circulation, this finding supports the fact that the patient is in remission. Moreover, since mature differentiated cells have a low proliferative capacity, there is a low risk for relapse.