TPOX is a short tandem repeat (STR) DNA locus having a four nucleotide repeat pattern (AATG). It is seen to have a greater frequency of tri-allelic genotypes than other STRs, although such genotypes are relatively rare overall. Here we describe a tri-allelic genotype found in the TPOX DNA locus of an engraftment monitoring baseline sample.DNA was extracted from a patient’s pre-transplant peripheral blood lymphocytes using the Qiagen Mini Blood Kit. This DNA was amplified on an Applied Biosystems 9700 thermocycler using Promega’s PowerPlex16 System human identity kit. The amplicon was run on an Applied Biosystems 3130xl Genetic Analyzer, and the data evaluated with GeneMapper ID-X software.The DNA profile appeared normal, except in the TPOX locus (2p23-2pter). TPOX presented an 8-9-10 genotype with the three alleles having similar intensity. This is one of two types of tri-allelic pattern that might be discerned, the other having two alleles with different intensity than the third. The observed pattern suggests a chromosomal rearrangement. Karyotype analysis showed no occurrence of 2p trisomy.We report a TPOX tri-allelic subject where the tri-allelic pattern is not the result of a visible 2p trisomy. This is concordant with other findings of allele 10 being the predominant extra TPOX allele and suggesting that it is transmitted on the X chromosome.