OFD or a metaphyseal fibrous defect ⁄ non-ossifying fibroma-the most likely diagnoses for tumour sited in the cortical bone. A haematoxylin and eosin-stained section of the needle core biopsy specimen showed the characteristic features of a chondroblastoma, despite the fact that the vast majority of chondroblastomas are intramedullary epiphyseal neoplasms. Many of the lesional cells were strongly and diffusely positive for CKs (extensively positive for MNF116, AE1 ⁄ AE3, CAM5.2, CK8 and CK18 and more focally positive for CK19) (Figure 2).Another diagnosis that should be considered in a case of an unusual CK+ skeletal neoplasm is an extraaxial chordoma ⁄ chordoma periphericum, a neoplasm which also expresses CKs, specifically CK19. 6 However, our CK+ intracortical neoplasm was negative for brachyury, a transcription factor which is crucial for notochordal development. We have recently found this molecule to be expressed in 100% of 52 axial chordomas analysed and not in over 300 other tumours apart from one intracortical tibial neoplasm. 2,3 This brachyury-positive extra-axial neoplasm exhibited characteristic, although not typical, morphology and typical immunohistochemistry (CK19, S100 and brachyury) of a chordoma. However, as it was not sited in the spinal column, we propose that it is more appropriate to classify such a neoplasm as one showing notochordal differentiation rather than a chordoma. Chordomas are considered to arise from notochordal remnants and it is unlikely that ectopic notochordal remnants account for the origin of this tumour.