SFRP2 and Ewing sarcoma: Besides the enhanced expression observed in metastatic OS, using the publically available R2 database (http://hgserver1.amc.nl/cgi-bin/r2/main.cgi), we investigated whether sFRP2 expression also has clinical relevance for sarcomas, as increased mRNA expression levels are significantly associated with poorer event free and overall survival for Ewing sarcoma, the second most common pediatric bone tumor (Additional file 1: Figure S1b and c).