Amplification of the CDK4 gene in sarcomas: tumor specificity and relationship with the RB gene mutation. Amplification of the CDK4 gene, which encodes a key molecule in the cell cycle, has been shown in some types of human neoplasms, including bone and soft tissue tumors. It is also reported that the CDK4 gene is coamplified with other sequences in the 12q13-15 region, including the MDM2 and SAS genes. Using 146 DNA samples derived from a variety of bone and soft tissue tumors, we have studied the pattern of amplification of these three genes, CDK4, MDM2, and SAS, to investigate whether there are any tumor type specific patterns of amplification. Amplification of at least one of these three genes was found in 18 tumors, and five different patterns of amplification were observed. Amplification of all of these three genes was detected in 9 cases. Amplification of the CDK4 gene without MDM2 amplification was observed in osteosarcomas and a chondrosarcoma but not in soft tissue tumors, whereas amplification of MDM2 gene alone was observed in malignant fibrous histiocytomas (MFHs), liposarcomas, and lipomas, but not in bone tumors. These results suggested that the CDK4 region is the primary target for amplification in bone tumors, whereas the MDM2 region is in soft tissue tumors. We also investigated the relationship of CDK4 amplification with retinoblastoma (RB) gene mutations in osteosarcomas, for which we have already performed the mutation analyses in detail. Interestingly, contrary to the prevailing theory that CDK4 amplification is an alternative mechanism for RB gene mutation, we found that three of four cases with amplification of the CDK4 gene showed loss of expression of the RB protein, one of which was proved to have an gross DNA alteration in the RB locus. This redundancy of mutations may indicate that the amplification of CDK4 may have some roles other than the inactivation of the RB protein in the development of osteosarcomas.