Assessment of chimerism after allogeneic bone marrow transplant frequently relies on PCR amplification of donor- and recipient-specific polymorphic short tandem repeats (STR) with subsequent fluorescent detection of amplicons. In vitro amplification of STR loci is often achieved through commercially available, multiplex PCR kits. While originally developed for forensic purposes, these kits are increasingly being used in clinical laboratories for early detection of graft failure and relapse of disease. Despite the obvious benefits of sensitivity, accuracy, reproducibility, and ease-of-use of these commercial kits, laboratories must understand the technological shortcomings of such assays to avoid misinterpretation of patient results. The current case illustrates how primer site polymorphisms associated with specific STR alleles can potentially lead to erroneous interpretation of engraftment analysis using one of these multiplex STR kits.