CD31 is expressed by stem cells of the hematopoietic system and is mainly used to identify and concentrate these cells for experimental studies as well as for bone marrow transplantation. Endothelial cells also express this marker; therefore, antibodies against CD31 have been used as a tool to identify the vascular origin of neoplasms.1-3 Anti-CD31 has been shown to be highly specific and sensitive for vascular endothelial cells.4 Staining of non-vascular tumors (excluding hematopoietic neoplasms) has not been observed.