**2.1 Introduction**

Quantitative monitoring of chimerism after allo-hSCT based on PCR amplification of microsatellite STR markers has become an important component of post-transplant surveillance of patients. Each STR marker is a system of many alleles, all sharing the basic structure of a repeat (2–8 bp in length), (4–5 bp in our study) but differing in the number of tandem repeats of this sequence. They can be applied for follow-up of virtually all patients and only small amounts of DNA are required for the test to estimate to donor/recipient chimerism after allo-hSCT [15–17]. Quantitative chimerism monitoring can document engraftment, predict graft failure or rejection, identify those patients who are at the highest risk to develop relapse and clarify the origin of the cells after relapse [18–23].
