Expression of the smooth muscle calponin gene in human osteosarcoma and its possible association with prognosis. The basic calponin gene is a smooth muscle differentiation-specific gene that encodes an actin-binding protein involved in the regulation of smooth muscle contractility. We studied the expression of the calponin gene in 8 human osteosarcoma cell lines and 17 primary human osteosarcoma tissues by RT-PCR analysis. We also analyzed mRNA expression of smooth muscle-specific genes including SM22alpha, caldesmon and alpha-actin, and for neutral and acidic calponin isoforms. The genes were expressed at various levels by osteosarcoma cell lines and tissues of diverse histological subtypes. The basic calponin protein of an expected size was detected in osteosarcoma cell lines by immunoblot analysis and was localized by immunohistochemistry in the cytoplasm of the tumor cells in osteosarcoma tissues. Survival was found to be significantly increased in patients whose tumors exhibited basic calponin expression, compared with those with no expression. Alterations in the expression of other markers examined were not correlated with prognosis. Our results suggest that the basic calponin gene product may be a novel prognostic variable in patients with osteosarcoma.