Altered expression of cell cycle regulatory proteins in benign and malignant bone and soft tissue neoplasms. The binding of cyclins to cyclin-dependent kinases regulates cell proliferation. Overexpression of cyclins is believed to deregulate the cell cycle in human tumors. Here the expression of G1 cyclins D1 and D3, and of Ki-67 in a variety of bone and soft tissue sarcomas was assessed as compared to adjacent normal tissue and to a subset of leiomyomas.Twenty-nine human bone and soft tissue sarcomas were evaluated. Tissue sections from each case were subjected to immunostaining for cyclin D1, cyclin D3 and Ki-67 using the avidin-biotin complex method.Cyclin D1 nuclear positivity was detected in 28% of sarcomas and in none of the leiomyomas. Cyclin D3 nuclear positivity was present in 62% of sarcomas and in none of the leiomyomas. Ki-67 nuclear staining was positive in 86% of sarcomas but in only 16% of leiomyomas. In addition, upregulation of cyclin D1 was observed in leiomyosarcomas, pleomorphic sarcomas and gastrointestinal stromal tumors, but not in liposarcomas or osteosarcomas. Cyclin D3, however, was expressed in all of the sarcoma types including 2 out of 5 liposarcomas and 1 out of 4 osteosarcomas. The normal soft tissue adjacent to the tumors when present (10 cases) was negative for cyclin D1 and D3, and expressed Ki-67 in 5% of the cell nuclei. The expression of cyclin D3 was also noted in human sarcoma cell lines (SKLMS, MG63, SaOS-2 and HT1080) by Western blot.The higher expression of cyclin D1 and D3 and of Ki-67 in bone and soft tissue sarcomas, as compared to leiomyomas and peritumoral normal soft tissue, suggests that high cyclin expression may contribute to deregulation of the cell cycle in bone and soft tissue tumors. These data suggest a role of cyclins in the process of human sarcomagenesis.