Nuclear p63 expression in osteoblastic tumors. Expression of the p63 tumor suppressor protein has been reported in the mononuclear stromal cells of giant cell tumor of the bone, which may represent osteoblast-precursor cells. Only a limited number of osteoblastic tumors have been studied for p63 expression thus far. We therefore examined whether p63 may serve as a marker for osteoblastic differentiation in osteosarcomas or as a differential diagnostic marker to distinguish osteoblastoma from osteosarcoma. Immunohistochemical stains for p63 were performed on a tissue microarray containing 71 chemotherapy naïve biopsy samples of osteosarcoma, 21 whole sections of osteosarcoma, and 8 osteoblastomas. Nuclear p63 was detected in seven of eight osteoblastomas but was restricted to stromal cells within primitive, immature-appearing areas of osteoid deposition. Although only 7 of 71 (10 %) biopsy samples of osteosarcoma represented on the tissue microarray were positive for p63, 7 of 21 (33 %) osteosarcomas were positive when whole tissue sections were evaluated. Although p63 is detected in most osteoblastomas, it is also observed in a significant subset of osteosarcomas, severely limiting its utility in distinguishing between benign and malignant osteoblastic tumors. The relatively low prevalence of p63 expression in osteosarcoma would also seem to preclude its use as a marker of osteoblastic differentiation in skeletal sarcomas.