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We report the status of the RBI, TP53, and MDM2 genes in human osteosarcomas and cell lines established from surgical specimens and transplanted into athymic naked mice. By using reverse transcriptase-polymerase chain reaction (RT-PCR) as a prescreening technique and posterior sequencing, we observe new mutations in the RBI gene, notably a duplication in tandem of exons 3 through 6. TP53 mutations appear in codons most frequently mutated in osteosarcomas. We have not seen MDM2 gene amplification in any reported case. These molecular alterations appear in different osteosarcomas not simultaneously present in the same tumor sample. A link has been described between these three genes in the pathways that control the cell cycle and the tumoral progression, but their functions are probably independent in the development of osteosarcomas. TP53 mutations appear in adult patients, whereas RB1 alterations occur mostly in younger patients.