1. Introduction
Allogeneic stem cell transplantation (alloSCT) is a widely applied treatment modality for both malignant and benign diseases. The goal of transplantation is to replace autologous hematopoiesis with stem cells harvested from the donor. However, in a proportion of patients after alloSCT autologous cells can still can be detected in peripheral blood (PB) or bone marrow (BM) [1]. The clinical significance of this phenomenon remains controversial. Increasing mixed chimerism may be associated with the high risk of relapse or rejection [2]. The decrease in the number of host cells may indicate a low risk of recurrence [3]. The consequence of the stable presence of a low-level autologous signal is still unclear. Some studies indicate that such circumstance may also be associated with a higher incidence of relapse [4], while others dispute such relationship [5]. Conflicting data results in different therapeutic approaches. It has become commonly accepted that hematopoietic chimerism monitoring may allow early introduction of immunotherapy aiming to prevent an imminent treatment failure [6]. However, there is no consensus as to whether sufficient evidence to support this approach has been provided [7] or further clinically oriented randomized studies are required [8]. New promising modification of alloSCT employing reduced intensity conditioning also relies on precise quantitative determination of the proportion of donor and host cells. DLI following infusion of stem cells after non-myeloablative conditioning intend to reduce the amount of autologous DNA and to convert the recipient to full donor chimera [9]. Such an approach is only applicable when quantitative measures to monitor chimerism are available.