BACKGROUND: Patient derived xenografts (PDXs) provide a unique opportunity for investigators to study tumor cell activity, response to therapeutics, and resistance patterns without exposing the human patient to experimental compounds, and thereby play a crucial role in pre-clinical evaluation of new therapies. It has been reported that PDXs may undergo a transformation to lymphoma, most commonly associated with Epstein Barr virus (EBV). If the character of a xenograft becomes compromised and remains undetected, it could have a detrimental impact on the research community as a whole. Our lab has established a number of pediatric solid tumor PDXs which accurately recapitulate the human tumors following several passages. One particular neuroblastoma PDX was noted to grow quickly and with an unusual phenotype, leading us to hypothesize that this PDX had undergone a transformation. METHODS: The PDX in question was investigated with histology, immunohistochemistry (IHC), EBER in situ hybridization, and PCR to determine its identity. RESULTS: Histology on the tumor revealed a small, round blue cell tumor similar to the original neuroblastoma from which it was derived. IHC staining showed that the tumor was composed of lymphocytes that were CD3 positive
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