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Ectopic and heterotopic bone formation rarely occur in stromal tissues of various epithelial tumors. In the present study, ectopic bone forming tumors; calcifying epithelioma Malherbe (6), renal cell carcinoma (1), thyroid adenocarcinoma (1) and ovary carcinoma (1) were examined. The process of ectopic bone formation could be classified into 1) bone formation related to epithelial tissue as found in calcifying epithelioma of Malherbe, 2) bone formation from perivascular smooth muscle, and 3) that due to metaplasia occurring in stromal connective tissue of epithelial tumors. Immunohistochemical detection of proteoglycans (PG) was made in bone-forming matrix areas, and these areas were positive for chondrotin 4 and 6 sulfate, and dermatan and heparan sulfate PGs. Immunohistochemically, fibronectin, collagen III and bone sialoprotein as matrix proteins, and carbonic anhydrase as calcification marker were tested and they appeared in these pre-calcifying tissues.