AbstractAlthough osteosarcoma is the most common bone malignancy, the molecular and cellular mechanisms influencing its pathogenesis have remained elusive. Prion proteins (PRNP and PRND), known mostly for its involvement in neurodegenerative spongiform encephalopathies, have been recently demonstrated to be involved in resistance to apoptosis, tumorigenesis, proliferation, and metastasis. The main aim of research was to study whether prion proteins were over‐expressed in human osteosarcoma, and if prion proteins could have a role also in osteosarcomas. We evaluated differential gene expression between 22 cases of osteosarcoma and 40 cases of normal bone specimens through cDNA microarray analysis spanning a substantial fraction of the human genome. PRNP and PRND are significantly over‐expressed in osteosarcoma. PRNP and PRND appear involved with some important genes related to tumorigenesis and apoptosis. PRNP is linked to PTK2, RBBP9, and TGFB1 while PRND is linked to TNFSF10, BCL2A1, NFKB2, and TP53RK. Increased expression on Affymetrix arrays of prion proteins seems to be associated with the development of osteosarcoma. Prions seem to induce a negative regulation of apoptosis, thus promoting osteosarcoma development and progression. Osteosarcoma is a very aggressive tumor and even after modern chemotherapy and excision of tumors efforts are needed to improve clinical outcome. Since Prion proteins seem to be related to osteosarcoma development, their inhibition could represent a new approach to the molecular treatment of osteosarcoma. © 2011 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 30:1004–1012, 2012