Homeobox B3, B4, and C6 gene product expression in osteosarcomas as detected by immunocytochemistry. Osteosarcoma (OS) is a malignant neoplastic disease of the bone, of mesenchymal origin and with considerable morphologic heterogeneity, consisting of malignant stoma with evidence of malignant osteoid, bone and/or cartilage production. The mammalian homeobox (HOX) represents a highly conserved DNA motif of 183 base pairs, encoding the 61 amino acid DNA-binding homeodomain, through which the HOX gene products regulate the transcription of other genes involved in onto- and histogenesis. Re-expression of HOX proteins has been identified in a wide variety of neoplastically transformed cell types and it seems that the HOX genes represent yet another family of oncofetal antigens involved in both normal development and oncogenesis, as well as tumor tissue progression. During this study, the expression pattern of three HOX gene products (HOX-C6, -B3, and -B4) was examined immunocytochemically in human osteosarcoma (OS) tissues. In all observed (16/16) OS cases, HOX-C6 was present in over 90% of the neoplastically transformed cells (+4), demonstrating a high to medium grade (A to B) staining intensity. Similar results were obtained in OS cells for the other two observed proteins (HOX-B3 and -B4; over 90% or +4 and a high to medium grade staining intensity or A and B). The significance of the expression of class I HOX proteins in the pathobiology, diagnosis and prognostication of human OS should be established by further investigations.