Abstract Hypothesis: Endosialin emerged recently as a potential marker and therapeutic target for adult and pediatric sarcoma. Given evidence of a possible common progenitor cell for mesenchymal and neural cell lineages, we wondered whether expression of endosialin may be shared by sarcomas and neuroblastomas, which are cancers of mesenchymal origin and neural crest origin, respectively. Methods: Endosialin protein expression was studied in live human neuroblastoma cell lines by flow cytometry using a fully human monoclonal antibody against human endosialin. Endosialin-positive human neuroblastoma cells were subsequently implanted at different anatomic sites in nu/nu mice and allowed to grow. Tumors were collected, formalin fixed and subjected to immunohistochemistry using a fully human monoclonal antibody against human endosialin. Using the same IHC assay, endosialin protein expression was also studied in formalin-fixed paraffin-embedded human clinical specimens of neuroblastoma. Results: We tested 10 human neuroblastoma cell lines for endosialin protein expression by flow cytometry and found that 9/10 expressed endosialin. Several of the positive cell lines were derived from bone marrow metastases, suggesting that endosialin expression is maintained in advanced disease. We modeled endosialin-positive neuroblastoma in vivo by implanting SK-N-AS cells in mice subcutaneously and in the subrenal capsule, kidney being a site where neuroblastoma sometimes originates. Immunohistochemical analysis revealed that SK-N-AS cells, which are positive for endosialin in vitro and derived from the bone marrow metastasis of an adrenal primary tumor, formed endosialin-positive subcutaneous tumors and endosialin-positive renal tumors, demonstrating that endosialin expression is supported by different microenvironments in various anatomic locations. Immunohistochemistry of human clinical specimens of neuroblastoma showed expression of endosialin. All specimens of neuroblastoma were bone marrow metastases, demonstrating that endosialin is expressed in advanced disseminated neuroblastoma. Conclusions: Our work demonstrates that endosialin expression is shared by sarcomas and neuroblastomas. The expression of endosialin in neuroblastoma potentially opens up a new therapeutic horizon for neuroblastoma patients, including those suffering from advanced disease. Citation Format: {Authors}. {Abstract title} abstract. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5251.